chore(.github/workflows): let librarian update workflow also sync new commit to generation_config.yaml (#13372)

Add this step temporarily so that this workflow also triggers hermetic
build generation workflow, and we can verify changes in the same PR.
This change is intended to be set till 06/10/2026 to verify librarian
generate against hermetic build.

Here is the sequence:
- nightly, this scheduled workflow updates librarian.yaml with new
googleapis commit.
- [additional step] update generation_config.yaml with same commit.
- run `librarian generate -all` and add code change to commit
- raise PR.
- hermetic build [generation
workflow](https://github.com/googleapis/google-cloud-java/blob/main/.github/workflows/hermetic_library_generation.yaml)
triggered because generation_config.yaml `googleapis_commitish` is
updated.

On the next day, we verify the results on this PR, expect hermetic build
[generation workflow] runs succesfully with no code change added. If
any, we should investigate the diff.

Note: hermetic build workflow will not trigger on this PR, because I
pushed from fork. This is to avoid it adding code changes to this PR.

Author: zhumin8

.github/workflows/update_librarian_googleapis.yaml

@@ -68,6 +68,11 @@ jobs:
         new_commit=$(go run "github.com/googleapis/librarian/cmd/librarian@${version}" config get sources.googleapis.commit)
         echo "new_commit=${new_commit}" >> $GITHUB_OUTPUT
         echo "short_commit=${new_commit:0:7}" >> $GITHUB_OUTPUT
+    # TODO(https://github.com/googleapis/librarian/issues/6220): Remove this step.
+    - name: Update generation_config.yaml
+      run: |
+        sed -i -e "s/^googleapis_commitish.*$/googleapis_commitish: ${{ steps.commit.outputs.new_commit }}/" generation_config.yaml
+        git diff generation_config.yaml
     - name: Detect Changes To Librarian.yaml
       id: detect_librarian
       run: |
@@ -127,7 +132,7 @@ jobs:
       env:
         GH_TOKEN: ${{ secrets.CLOUD_JAVA_BOT_GITHUB_TOKEN }}
         PR_TITLE: "chore: update googleapis commitish to ${{ steps.commit.outputs.short_commit }}"
-        PR_BODY: "Updated googleapis commitish in librarian.yaml to https://github.com/googleapis/googleapis/commit/${{ steps.commit.outputs.new_commit }}"
+        PR_BODY: "Updated googleapis commitish in librarian.yaml and generation_config.yaml to https://github.com/googleapis/googleapis/commit/${{ steps.commit.outputs.new_commit }}"
       run: |
         set -x