fix: adjust workflow link for pull_request_target workflow runs (#12734)

Fixes #12660, reported by @mahlzahn.

### Tests for Go changes

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [ ] <s>`make pr-go` before pushing</s> because the OOM killer kills it before it does anything useful

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12734
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>

routers/web/repo/actions/view.go

@@ -63,6 +63,15 @@ func View(ctx *app_context.Context) {
 		return
 	}
 
+	workflowDefinitionCommitSHA := job.Run.CommitSHA
+	// if the trigger event is `pull_request_target`, then the definition of the workflow is taken
+	// from the base branch instead of the commit the workflow is triggered on
+	if job.Run.TriggerEvent == actions.GithubEventPullRequestTarget {
+		if pullPayload, err := job.Run.GetPullRequestEventPayload(); err == nil && pullPayload.PullRequest != nil && pullPayload.PullRequest.Base != nil {
+			workflowDefinitionCommitSHA = pullPayload.PullRequest.Base.Sha
+		}
+	}
+
 	workflowName := job.Run.WorkflowID
 
 	ctx.Data["RunIndex"] = runIndex
@@ -72,7 +81,7 @@ func View(ctx *app_context.Context) {
 	ctx.Data["AttemptNumber"] = attemptNumber
 	ctx.Data["WorkflowName"] = workflowName
 	ctx.Data["WorkflowURL"] = ctx.Repo.RepoLink + "/actions?workflow=" + workflowName
-	ctx.Data["WorkflowSourceURL"] = ctx.Repo.RepoLink + "/src/commit/" + job.Run.CommitSHA + "/" + job.Run.WorkflowPath()
+	ctx.Data["WorkflowSourceURL"] = ctx.Repo.RepoLink + "/src/commit/" + workflowDefinitionCommitSHA + "/" + job.Run.WorkflowPath()
 
 	viewResponse := getViewResponse(ctx, &ViewRequest{}, runIndex, jobIndex, attemptNumber)
 	if ctx.Written() {