Add support for adding ghcup release channel (-> GHC pre-releases)

[robx]

(Reference: #107.)

ghcup supports installing non-standard GHC releases via so-called "release channels": https://www.haskell.org/ghcup/guide/#pre-release-channels

I'd like to propose an input to this action for adding an extra release channel. I.e.:

inputs:
  ...
  ghcup-release-channel:
    required: false
    description: "A release channel to add to ghcup via `ghcup add-release-channel`."

If set, this would mean that the action calls

ghcup add-release-channel ${ghcup-release-channel}

before the call to ghcup install.

This would add support for GHC pre-releases by allowing users to add the release channel managed at https://github.com/haskell/ghcup-metadata/blob/master/ghcup-prereleases-0.0.7.yaml.

(Ideally, the GHC HEAD support (#93) could be folded into this too at some point -- the nice thing about the ghcup metadata is that it takes care of that deb9/deb10/deb11 confusion.)

[hazelweakly]

I like this a lot, and I'm definitely in favor of doing it. Let me see how much energy I have for this this weekend