fix docker executor on windows and local actions (#34)

If the Workdir field doesn't ends with the filepath seperator,
bad things happen

Fixes #33

Sample for host mode on windows, needs be adjusted for linux e.g. replace pwsh with bash
Also fixes
```yaml
on: push
jobs:
  _:
    runs-on: self-hosted
    steps:
    - uses: actions/checkout@v3
      with:
        path: subdir/action
    - uses: ./subdir/action
```

with an action.yml in the same repo
```yaml
runs:
  using: composite
  steps:
    - run: |
        echo "Hello World"
      shell: pwsh
```

Co-authored-by: Christopher Homberger <christopher.homberger@web.de>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/34
Reviewed-by: Jason Song <i@wolfogre.com>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: ChristopherHX <christopherhx@noreply.gitea.io>
Co-committed-by: ChristopherHX <christopherhx@noreply.gitea.io>

runtime/task.go

@@ -209,7 +209,7 @@ func (t *Task) Run(ctx context.Context, task *runnerv1.Task) (lastErr error) {
 
 	input := t.Input
 	config := &runner.Config{
-		Workdir:               "/" + preset.Repository,
+		Workdir:               "." + string(filepath.Separator),
 		BindWorkdir:           false,
 		ReuseContainers:       false,
 		ForcePull:             input.forcePull,