bios cradle: Allow `with-ghc` to be specified programmatically

[sol]

Similar to how you can specify dependencies programmatically by writing to $HIE_BIOS_DEPS I would like to be able to specify with-ghc programmatically by writing to e.g. $HIE_BIOS_WITH_GHC.

[fendor]

Unfortunately, what you are proposing is too late for HLS. The GHC version is cradle wide, but what you are describing is per Component. See #274 for a discussion.

[sol]

@fendor just to make sure that I understand your point correctly, are you saying that instead of

cradle:
  bios:
    shell: ... # determine options and path to ghc

we would need something like

cradle:
  bios:
    shell: ... # determine options
    with-ghc:
      shell: ... # determine path to ghc

(That is, we need the user to specify two separate commands, one for options, and one for the GHC version.).

Is that where you are heading at?

[fendor]

Yes, exactly!