Allow provider to use ~/.config/glab-cli/config.yml in addition to GITLAB_TOKEN variable

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In other providers (eg: AWS or azure), the provider accepts any authentication mechanisms that are supported by the SDK. In the cases of AWS and azure, they can use both environment variables, or the files created in ~/.aws or ~/.azure. In the case of gitlab, "glab auth login" via OAuth/web creates ~/.config/glab-cli/config.yml, but there doesn't seem to be a way for the provider to pick that up. This means that we need to externally obtain it via yq. It would be much easier and consistent to allow the provider to use ~/.config/glab-cli/config.yml if (1) no environment variable is defined, or (2) it is defined as a property.