Auto merge of #157007 - Kobzol:reject-invalid-backend, r=jieyouxu

Reject invalid codegen backends on CI

The way we set codegen backends in CI jobs contained several bugs.

1) We didn't check if the specified codegen backend is valid. That can be useful in forks, but not on our CI, there it should be a loud error.
2) The codegen backends were specified with weird quoting that was (I think) producing invalid backends.
3) The specified backends in `RUST_CONFIGURE_ARGS` were being overridden due to `run.sh` setting codegen backends based on the `CODEGEN_BACKENDS` environment variable (see https://github.com/rust-lang/rust/pull/156989).

This PR should fix all of these.

After doing some CI runs on this PR, I realized that setting `CODEGEN_BACKENDS` from within the Dockerfile doesn't work, because we use `docker run --env CODEGEN_BACKENDS`, and that will unset the inner env even if the same env is not actually configured in the environment from where `docker run` is executed (sigh). So instead I decided to pass the env variables from `jobs.yml`. It's not really pretty, but it is a solution that should work with the current structure.