.gitattributes: make github index files in 'build' dirs

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

.gitattributes

@@ -2,6 +2,13 @@
 BUILD linguist-language=Python
 site/* linguist-documentation
 
+# Github excludes files in 'build' directory from their search by default.
+# This negates the exclusion.
+#
+# For more information, see:
+#   https://docs.github.com/en/search-github/searching-on-github/finding-files-on-github#customizing-excluded-files
+**/build/** linguist-generated=false
+
 # Files that should not use CRLF line endings, even on Windows.
 tools/genrule/genrule-setup.sh -text