feat: only block cask install on Linux

[SMillerDev]

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This is the first step to allowing font (and later binary) casks to be installable on Linux.
TODO:

• Think about how to handle brew search --cask X
• Think about how to handle brew info --cask X

[MikeMcQuaid]

Makes sense so far! Probably want to make --cask explicitly required on Linux until this is (much) further along?

[SMillerDev]

Probably want to make --cask explicitly required on Linux until this is (much) further along?

You mean for install? I'll have to check, but I think you have to manually tap the cask repo for this to do anything on Linux for now. That seems like a sufficient barrier.

[MikeMcQuaid]

You mean for install? I'll have to check, but I think you have to manually tap the cask repo for this to do anything on Linux for now. That seems like a sufficient barrier.

@SMillerDev Ok, as long as brew install docker or brew install google-chrome on Linux retains the same behaviour as before 👍🏻

[SMillerDev]

I'll do an extra check for those specific ones.

What do you think should happen to search and info? Just allow them for casks?

[MikeMcQuaid]

What do you think should happen to search and info? Just allow them for casks?

I think probably require --cask or something?

[SMillerDev]

Tested this in a codespace, it has the same result, but a different output.

Before this PR:

linuxbrew@codespaces-069e0c:~/.linuxbrew/Homebrew$ brew install google-chrome
==> Downloading https://formulae.brew.sh/api/formula.jws.json

Warning: No available formula with the name "google-chrome". Did you mean google-benchmark?
==> Searching for similarly named formulae...
==> Formulae
google-benchmark

To install google-benchmark, run:
  brew install google-benchmark

After this PR:

linuxbrew@codespaces-069e0c:~/.linuxbrew/Homebrew$ brew install google-chrome
==> Downloading https://formulae.brew.sh/api/cask.jws.json
##O=#    #                                                                                                                                         
Error: macOS is required for this software.

[MikeMcQuaid]

@SMillerDev What about brew install --cask google-chrome

I think the prior behaviour is better on Linux for now, sorry, but I think if you added --cask then the latter would make sense.

[SMillerDev]

I'll reinstate that. Seems like I was too eager with the CLI changes. After that and fixing the tests, are you fine to merge this while I work on the font installing? Or you'd rather bundle it and merge in one go?

[MikeMcQuaid]

@SMillerDev I'm game to merge smaller/incremental changes here, yeh 👍🏻

[SMillerDev]

Okay, with this latest change that behaviour should be reinstated.

[SMillerDev]

Looks like all checks are green and the behavior was restored as requested.
@MikeMcQuaid do you mind taking another look?

[MikeMcQuaid]

Looks good, thanks @SMillerDev!

[apainintheneck]

Great work here!