build: add cab-token-generator to always install deps (#12851)

Adds 'google-auth-library-java/cab-token-generator' to the
`always_install_deps_list` in `.kokoro/common.sh`.

After the monorepo migration, release PRs failed the
`gapic-generator-java-bom` check because they couldn't resolve the new
auth library version from Maven Central before it was published.

This fix ensures the artifact is installed to the local Maven repository
during the CI's install phase, satisfying the canary validation without
requiring a full repository build.

Fixes #12843

Author: meltsufin

.kokoro/common.sh

@@ -408,6 +408,7 @@ function install_modules() {
     always_install_deps_list=(
       'google-auth-library-java/appengine'
       'google-auth-library-java/bom'
+      'google-auth-library-java/cab-token-generator'
       'google-auth-library-java/credentials'
       'google-auth-library-java/oauth2_http'
       'sdk-platform-java/java-shared-dependencies'