Increase timeout of macos artifact build to 4 hours (#26738)

tools/internal_ci/macos/grpc_build_artifacts.cfg

@@ -17,7 +17,7 @@
 # Location of the continuous shell script in repository.
 build_file: "grpc/tools/internal_ci/macos/grpc_build_artifacts.sh"
 gfile_resources: "/bigstore/grpc-testing-secrets/gcp_credentials/GrpcTesting-d0eeee2db331.json"
-timeout_mins: 150
+timeout_mins: 240
 action {
   define_artifacts {
     regex: "**/*sponge_log.*"