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

[zhumin8]

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 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.

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[zhumin8]

dry run updates commit as expected: https://screenshot.googleplex.com/4JrQaWBWzaMKCLh