Resolve "spec/lib/gitlab/backend/shell_spec.rb causes other specs to fail"

## What does this MR do?

It ensures we rely on `Gitlab::Shell`'s public API and not on its internal. Since `Gitlab::Shell` is caching the value of its token, the only way to get the correct token is to call `Gitlab::Shell.secret_token`, not to read the `Gitlab.config.gitlab_shell.secret_file` file!

Fixes #24599

See merge request !7557