GCE_METADATA_HOST not passed through to credential-helper

[dziemba]

When using docker-credential-gcr with regular docker, setting the GCE_METADATA_HOST environment variable tells it to use the given endpoint instead of the default one to fetch tokens via ADC.

This does not work with rules_oci since the environment variable is not passed on to the credential helper process.

I believe the trivial fix would be to just add it to this list:
https://github.com/bazel-contrib/rules_oci/blob/a195e365/oci/private/authn.bzl#L369

Is that correct? Would it be acceptable to add it there? If yes, I'm happy to create a PR for it.
If that use-case is too specific and you'd rather not add the env to the list, is there any other way to expose the correct settings, i.e. make the list of pass-thru-envs dynamic?