chore(ci): run linter on proto and grpc modules when librarian.yaml is updated

[zhumin8]

This change aim to catch future lint issues when onboarding libraries at the initial PR, while not offsetting regular PR's lint run time.

For googleapis/librarian#7146

[gemini-code-assist]

Code Review

This pull request modifies .kokoro/build.sh to conditionally include proto-* and grpc-* modules in the lint check when librarian.yaml is modified. The review feedback suggests improving the robustness of the file detection by using a regular expression to match exact file names and simplifying the complex conditional logic in the directory filtering using idiomatic Bash syntax.

[gemini-code-assist]

Using grep -q "librarian.yaml" can lead to false positives if there are other files containing librarian.yaml as a substring (e.g., not_librarian.yaml or some_librarian.yaml). To ensure we only match files named exactly librarian.yaml (either at the root or in a subdirectory), use a regular expression that matches the start of a line or a slash, followed by librarian.yaml at the end of the line.

Suggested change

	if grep -q "librarian.yaml" <<< "${changed_file_list}"; then
	if grep -qE "(^|/)librarian\.yaml$" <<< "${changed_file_list}"; then

[gemini-code-assist]

Using { [ ... ] || [[ ... ]]; } is complex and mixes different test syntaxes ([ and [[) along with command grouping { ... }. Since Bash's [[ ... ]] natively supports the || operator, you can simplify these conditions to be much more readable and idiomatic.

Suggested change

	{ [ "${has_librarian_change}" == "true" ] || [[ "$(basename "${dir}")" != "proto-google-"* ]]; } && \
	{ [ "${has_librarian_change}" == "true" ] || [[ "$(basename "${dir}")" != "grpc-google-"* ]]; } && \
	[[ "${has_librarian_change}" == "true" || "$(basename "${dir}")" != "proto-google-"* ]] && \
	[[ "${has_librarian_change}" == "true" || "$(basename "${dir}")" != "grpc-google-"* ]] && \