fix: data race condition causing incorrect shell on a task step if it referenced a matrix variable (#865)

The `setupShell` function would update the shell stored on the `Step` object, setting it to either a default value from the job, an expression evaluated in the context of the job, a default from the workflow, or finally falling back to bash or powershell defaults.  Typically this would be fine -- although it would trigger the data race detector because the `Step` is a shared object between multiple concurrent matrix evaluations for the job.

In the *really quite unlikely* case that the `shell` field on a step or job referenced a matrix variable, this data race would actually trigger the shared step's `Shell` value to end up as "whichever one was evaluated last", causing the wrong shell to be used.  The new `matrix-shell` test triggers this behavior, and fails without the associated code fix.

As a fix, the `Shell` field in `Step` is never mutated; instead only the value on non-shared `stepRun` instance is updated from `setupShellCommand`.  `Shell` was renamed to `RawShell` as part of verifying all references were updated and it seemed to make sense to keep that name since it is a pre-evaluator value.

```
==================
WARNING: DATA RACE
Write at 0x00c00013e9b0 by goroutine 1470:
  code.forgejo.org/forgejo/runner/v9/act/runner.(*stepRun).setupShell()
      /.../forgejo-runner/act/runner/step_run.go:210 +0x8f2
  code.forgejo.org/forgejo/runner/v9/act/common/git.FindGitRevision()
      /.../forgejo-runner/act/common/git/git.go:58 +0xc4
  code.forgejo.org/forgejo/runner/v9/act/model.(*GithubContext).SetSha()
      /.../forgejo-runner/act/model/github_context.go:161 +0x6b5
  code.forgejo.org/forgejo/runner/v9/act/runner.(*RunContext).getGithubContext()
      /.../forgejo-runner/act/runner/run_context.go:1228 +0x26ca
  ...

Previous write at 0x00c00013e9b0 by goroutine 1469:
  code.forgejo.org/forgejo/runner/v9/act/runner.(*stepRun).setupShell()
      /.../forgejo-runner/act/runner/step_run.go:210 +0x8f2
  code.forgejo.org/forgejo/runner/v9/act/common/git.FindGitRevision()
      /.../forgejo-runner/act/common/git/git.go:58 +0xc4
  code.forgejo.org/forgejo/runner/v9/act/model.(*GithubContext).SetSha()
      /.../forgejo-runner/act/model/github_context.go:161 +0x6b5
  code.forgejo.org/forgejo/runner/v9/act/runner.(*RunContext).getGithubContext()
      /.../forgejo-runner/act/runner/run_context.go:1228 +0x26ca

...
==================
```

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- bug fixes
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/865): <!--number 865 --><!--line 0 --><!--description Zml4OiBkYXRhIHJhY2UgY29uZGl0aW9uIGNhdXNpbmcgaW5jb3JyZWN0IGBzaGVsbGAgb24gYSB0YXNrIHN0ZXAgaWYgaXQgcmVmZXJlbmNlZCBhIG1hdHJpeCB2YXJpYWJsZQ==-->fix: data race condition causing incorrect `shell` on a task step if it referenced a matrix variable<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/865
Reviewed-by: earl-warren <earl-warren@noreply.code.forgejo.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

act/runner/runner_test.go

@@ -283,6 +283,7 @@ func TestRunner_RunEvent(t *testing.T) {
 		{workdir, "matrix", "push", "", platforms, secrets},
 		{workdir, "matrix-include-exclude", "push", "", platforms, secrets},
 		{workdir, "matrix-exitcode", "push", "Job 'test' failed", platforms, secrets},
+		{workdir, "matrix-shell", "push", "", platforms, secrets},
 		{workdir, "commands", "push", "", platforms, secrets},
 		{workdir, "workdir", "push", "", platforms, secrets},
 		{workdir, "defaults-run", "push", "", platforms, secrets},