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 🙏.
Meriadec Pillet