Currently Github would exclude files under 'build' dirs in the repo.
For example, try searching for
`third_party/googleapis/google/devtools/build` would yield no results,
but `third_party/googleapis/google/devtools/cloudbuild/` would works.

Fix this with a custom gitattributes.

Closes #22942.

PiperOrigin-RevId: 649114629
Change-Id: Ia62a447ce390671860c48a20352ac352332b88e3