chore: follow up fix to verify_library_generation (#2721)

#2716 didn't quite
fix what it should have. Follow up to fix the error:

```
Previous HEAD position was e54601f Merge 30c7c5b into 5d4567f
Switched to a new branch 'main'
branch 'main' set up to track 'origin/main'.
+ git fetch --no-tags --prune origin +main:refs/remotes/origin/main
From https://github.com/googleapis/sdk-platform-java
 - [deleted]             (none)     -> origin/main
+ git checkout renovate/markupsafe-2.x
error: pathspec 'renovate/markupsafe-2.x' did not match any file(s) known to git
```

---------

Co-authored-by: Joe Wang <[email protected]>

.github/workflows/verify_library_generation.yaml

@@ -16,13 +16,17 @@ jobs:
       shell: bash
       run: |
         set -ex
-        # Checkout a detached head, and then fetch the base ref to populate the detached head.
-        git checkout "${base_ref}"
-        git fetch --no-tags --prune origin +${base_ref}:refs/remotes/origin/${base_ref}
-        # Checkout a detached head, and then fetch the head ref to populate the detached head.
-        git checkout "${head_ref}"
-        git fetch --no-tags --prune origin +${head_ref}:refs/remotes/origin/${head_ref}
-        changed_directories="$(git diff --name-only ${base_ref} ${head_ref})"
+        # PRs that come from a fork need to be handled differently
+        if [[ ${head_repo_name} == ${base_repo} ]]; then
+          git checkout ${base_ref}
+          git checkout ${head_ref}
+          changed_directories="$(git diff --name-only ${base_ref} ${head_ref})"
+        else
+          git remote add fork ${head_repo_url}
+          git fetch fork # create a mapping of the fork
+          git checkout -b "${head_ref}" fork/${head_ref}
+          changed_directories="$(git diff --name-only "fork/${head_ref}" "origin/${base_ref}")"
+        fi
         if [[ ${changed_directories} =~ "library_generation/" ]]; then
           echo "should_run=true" >> $GITHUB_OUTPUT
         else
@@ -31,6 +35,9 @@ jobs:
       env:
         base_ref: ${{ github.event.pull_request.base.ref }}
         head_ref: ${{ github.event.pull_request.head.ref }}
+        head_repo_url: ${{ github.event.pull_request.head.repo.html_url }}
+        head_repo_name: ${{ github.event.pull_request.head.repo.full_name  }}
+        base_repo: ${{ github.repository }}
   library-generation-integration-tests:
     runs-on: ubuntu-22.04
     needs: should-run-library-generation-tests