Handle tags containing "@" character in `buildSLSAProvenancePredicate`

When using some monorepo-related tools (like [changesets](https://github.com/changesets/changesets)),
the produced tags have a special format that includes `@` character.

For example, a `foo` package on a monorepo will produce Git tags looking
like `foo@1.0.0` if using changesets.

When used in combination with `actions/attest-build-provenance`, the
action was not properly re-crafting the tag in `buildSLSAProvenancePredicate` because
it was always splitting the workflow ref by `@` and taking the second
element.

This result in this error on CI:

```
Error: Error: Failed to persist attestation: Invalid Argument - values do not match: refs/tags/foo != refs/tags/foo@1.0.0 - https://docs.github.com/rest/repos/repos#create-an-attestation
````

This PR slightly update the logic there, and rather take "everything
located after the first '@'". This shouldn't introduce any breaking
change, while giving support for custom tags.

I've added the corresponding test case, it passes, however I couldn't
successfully run the full test suite (neither on `main`). Looking
forward for CI outcome.

Thanks in advance for the review 🙏.

packages/attest/__tests__/__snapshots__/provenance.test.ts.snap

@@ -1,5 +1,47 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`provenance functions buildSLSAProvenancePredicate handle tags including "@" character 1`] = `
+{
+  "params": {
+    "buildDefinition": {
+      "buildType": "https://actions.github.io/buildtypes/workflow/v1",
+      "externalParameters": {
+        "workflow": {
+          "path": ".github/workflows/main.yml",
+          "ref": "foo@1.0.0",
+          "repository": "https://foo.ghe.com/owner/repo",
+        },
+      },
+      "internalParameters": {
+        "github": {
+          "event_name": "push",
+          "repository_id": "repo-id",
+          "repository_owner_id": "owner-id",
+          "runner_environment": "github-hosted",
+        },
+      },
+      "resolvedDependencies": [
+        {
+          "digest": {
+            "gitCommit": "babca52ab0c93ae16539e5923cb0d7403b9a093b",
+          },
+          "uri": "git+https://foo.ghe.com/owner/repo@refs/heads/main",
+        },
+      ],
+    },
+    "runDetails": {
+      "builder": {
+        "id": "https://foo.ghe.com/owner/workflows/.github/workflows/publish.yml@main",
+      },
+      "metadata": {
+        "invocationId": "https://foo.ghe.com/owner/repo/actions/runs/run-id/attempts/run-attempt",
+      },
+    },
+  },
+  "type": "https://slsa.dev/provenance/v1",
+}
+`;
+
 exports[`provenance functions buildSLSAProvenancePredicate returns a provenance hydrated from an OIDC token 1`] = `
 {
   "params": {

packages/attest/__tests__/provenance.test.ts

@@ -33,15 +33,7 @@ describe('provenance functions', () => {
     runner_environment: 'github-hosted'
   }
 
-  beforeEach(async () => {
-    process.env = {
-      ...originalEnv,
-      ACTIONS_ID_TOKEN_REQUEST_URL: `${issuer}${tokenPath}?`,
-      ACTIONS_ID_TOKEN_REQUEST_TOKEN: 'token',
-      GITHUB_SERVER_URL: 'https://foo.ghe.com',
-      GITHUB_REPOSITORY: claims.repository
-    }
-
+  const mockIssuer = async (claims: jose.JWTPayload): Promise<void> => {
     // Generate JWT signing key
     const key = await jose.generateKeyPair('PS256')
 
@@ -60,6 +52,18 @@ describe('provenance functions', () => {
 
     // Mock OIDC token endpoint for populating the provenance
     nock(issuer).get(tokenPath).query({audience}).reply(200, {value: jwt})
+  }
+
+  beforeEach(async () => {
+    process.env = {
+      ...originalEnv,
+      ACTIONS_ID_TOKEN_REQUEST_URL: `${issuer}${tokenPath}?`,
+      ACTIONS_ID_TOKEN_REQUEST_TOKEN: 'token',
+      GITHUB_SERVER_URL: 'https://foo.ghe.com',
+      GITHUB_REPOSITORY: claims.repository
+    }
+
+    await mockIssuer(claims)
   })
 
   afterEach(() => {
@@ -71,6 +75,16 @@ describe('provenance functions', () => {
       const predicate = await buildSLSAProvenancePredicate()
       expect(predicate).toMatchSnapshot()
     })
+
+    it('handle tags including "@" character', async () => {
+      nock.cleanAll()
+      await mockIssuer({
+        ...claims,
+        workflow_ref: 'owner/repo/.github/workflows/main.yml@foo@1.0.0'
+      })
+      const predicate = await buildSLSAProvenancePredicate()
+      expect(predicate).toMatchSnapshot()
+    })
   })
 
   describe('attestProvenance', () => {

packages/attest/src/provenance.ts

@@ -30,9 +30,11 @@ export const buildSLSAProvenancePredicate = async (
   // Split just the path and ref from the workflow string.
   // owner/repo/.github/workflows/main.yml@main =>
   //   .github/workflows/main.yml, main
-  const [workflowPath, workflowRef] = claims.workflow_ref
+  const [workflowPath, ...workflowRefChunks] = claims.workflow_ref
     .replace(`${claims.repository}/`, '')
     .split('@')
+  // Handle case where tag contains `@` (e.g: when using changesets in a monorepo context),
+  const workflowRef = workflowRefChunks.join('@')
 
   return {
     type: SLSA_PREDICATE_V1_TYPE,