feature: accept authTokenFile in addition to authToken

[colemickens]

I'm aiming towards using Home-Manager to manage my CI environment, with sops-nix's home-manager in tow.

As a result, I can consolidate all of my CI secrets into sops-encrypted data stored in the repo, and can use a single GitHub secret to unlock whatever a particular job needs and is assigned in its HM configuration.

However, from skimming the code and the docs, it doesn't seem that this action has a way to load the auth token from a file. Would y'all be willing to add this?

Thanks!

[colemickens]

I rather bluntly hacked something together that seems to be working for me: https://github.com/colemickens/github-action-install-tailscale/tree/main