[Test] Install g++ before gen_upb_api (#36628)

This is to address the following error error thrown when running protobuf-at-head. This is because Bazel needs a C++ compiler to run `tools/codegen/core/gen_upb_api.sh`.

```
Auto-Configuration Error: Cannot find gcc or CC (clang); either correct your path or set the CC environment variable
```

This is not ideal as installing `build-essential` package takes time but let's fix it first and get this installed in the CI image later.

Closes #36628

COPYBARA_INTEGRATE_REVIEW=#36628 from veblush:p-fix-at-head 01ac20f
PiperOrigin-RevId: 634114867

tools/internal_ci/linux/grpc_build_submodule_at_head.sh

@@ -74,6 +74,8 @@ then
   # update upb
   rm -rf third_party/upb/upb
   cp -r third_party/protobuf/upb third_party/upb
+  # generate upb gen source codes
+  export CC=gcc
   tools/codegen/core/gen_upb_api.sh
   # update utf8_range
   rm -rf third_party/utf8_range