google-cloud-cpp: add comment about doInstallCheck and dependencies

pkgs/development/libraries/google-cloud-cpp/default.nix

@@ -48,6 +48,10 @@ stdenv.mkDerivation rec {
     ninja
     pkg-config
   ] ++ lib.optionals (!doInstallCheck) [
+    # enable these dependencies when doInstallCheck failse because we're
+    # unconditionally building tests and benchmarks
+    #
+    # when doInstallCheck is true, these deps are added to installCheckInputs
     gbenchmark
     gtest
   ];