add a key to ctx.action dict to prevent protoc losing the default env

protobuf.bzl

@@ -90,6 +90,7 @@ def _proto_gen_impl(ctx):
         arguments=args + import_flags + [s.path for s in srcs],
         executable=ctx.executable.protoc,
         mnemonic="ProtoCompile",
+        use_default_shell_env=True,
     )
 
   return struct(