chore: fix verify_library_generation.yaml for forks (#2716)

PRs opened by renovate-bot are failing `verify_library_generation`
because renovate-bot opens PRs from a fork rather than main.

For instance #2657
is failing:

```
env:
    base_ref: main
    head_ref: renovate/markupsafe-2.x
+ git checkout main
Previous HEAD position was d84e607 Merge 645951c into 766646a[13](https://github.com/googleapis/sdk-platform-java/actions/runs/8914757209/job/24482995077?pr=2657#step:3:13)3d1465dca
Switched to a new branch 'main'
branch 'main' set up to track 'origin/main'.
+ git checkout renovate/markupsafe-2.x
error: pathspec 'renovate/markupsafe-2.x' did not match any file(s) known to git
```
This updates the yaml file to explicitly fetch specific branches which
should fix the issue.

---------

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

.github/workflows/verify_library_generation.yaml

@@ -21,8 +21,10 @@ jobs:
       shell: bash
       run: |
         set -ex
-        git checkout "${base_ref}"
-        git checkout "${head_ref}"
+        git checkout "${base_ref}" # Checkout a detached head, and then fetch the base ref to populate the detached head.
+        git fetch --no-tags --prune origin +${base_ref}:refs/remotes/origin/${base_ref}
+        git checkout "${head_ref}" # Checkout a detached head, and then fetch the head ref to populate the detached head.
+        git fetch --no-tags --prune origin +${head_ref}:refs/remotes/origin/${head_ref}
         changed_directories="$(git diff --name-only ${base_ref} ${head_ref})"
         if [[ ${changed_directories} =~ "library_generation/" ]]; then
           echo "should_run=true" >> $GITHUB_OUTPUT