Fix rvm ruby install failure (#27538)

See https://source.cloud.google.com/results/invocations/9a2a3ca2-72c9-43ef-af8c-fb1c8e7b488d/targets/grpc%2Fcore%2Fpull_request%2Fmacos%2Fgrpc_basictests_ruby/log

```
++ rvm get stable
Downloading https://get.rvm.io
Could not download rvm-installer, please report to https://github.com/rvm/rvm/issues
```

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -58,6 +58,9 @@ fi
 
 if [ "${PREPARE_BUILD_INSTALL_DEPS_RUBY}" == "true" ]
 then
+  # Workaround for https://github.com/rvm/rvm/issues/5133
+  export CURL_CA_BUNDLE=$(pwd)/etc/roots.pem
+
   # Fetch keys per https://rvm.io/rvm/install
   gpg_recv_keys_success=0
   for ((i=0;i<5;i++)); do