brew bundle support for Linux casks

[caycehouse]

Currently when trying to install a cask on Linux through brew bundle you are met with the following warning:

Skipping cask font-fira-code-nerd-font (on Linux)

This package does install successfully, when installed directly via brew install --cask:

\u276f brew install font-fira-code-nerd-font --cask
==> Downloading https://formulae.brew.sh/api/cask.jws.json
==> Downloading https://github.com/ryanoasis/nerd-fonts/releases/download/v3.3.0/FiraCode.zip
Already downloaded: /home/caycehouse/.cache/Homebrew/downloads/affcf9d46c77a9a85288ab0dee1ef9e183f97963290801fbb0146e4f99a0e197--FiraCode.zip
==> Installing Cask font-fira-code-nerd-font
Warning: macOS's Gatekeeper has been disabled for this Cask
==> Moving Font 'FiraCodeNerdFontPropo-SemiBold.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontPro
==> Moving Font 'FiraCodeNerdFont-Light.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFont-Light.ttf'
==> Moving Font 'FiraCodeNerdFont-Medium.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFont-Medium.tt
==> Moving Font 'FiraCodeNerdFont-Regular.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFont-Regular.
==> Moving Font 'FiraCodeNerdFont-Retina.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFont-Retina.tt
==> Moving Font 'FiraCodeNerdFont-SemiBold.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFont-SemiBol
==> Moving Font 'FiraCodeNerdFontMono-Bold.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontMono-Bol
==> Moving Font 'FiraCodeNerdFontMono-Light.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontMono-Li
==> Moving Font 'FiraCodeNerdFontMono-Medium.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontMono-M
==> Moving Font 'FiraCodeNerdFontMono-Regular.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontMono-
==> Moving Font 'FiraCodeNerdFontMono-Retina.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontMono-R
==> Moving Font 'FiraCodeNerdFontMono-SemiBold.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontMono
==> Moving Font 'FiraCodeNerdFontPropo-Bold.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontPropo-B
==> Moving Font 'FiraCodeNerdFontPropo-Light.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontPropo-
==> Moving Font 'FiraCodeNerdFontPropo-Medium.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontPropo
==> Moving Font 'FiraCodeNerdFontPropo-Regular.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontProp
==> Moving Font 'FiraCodeNerdFontPropo-Retina.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFontPropo
==> Moving Font 'FiraCodeNerdFont-Bold.ttf' to '/home/caycehouse/.local/share/fonts/FiraCodeNerdFont-Bold.ttf'
\U0001f37a  font-fira-code-nerd-font was successfully installed!

Also, when running brew bundle dump --cask with the previous cask installed, nothing is added to the Brewfile, removing the following check allows installed casks to be written:

homebrew-bundle/lib/bundle/extend/os/linux/bundle.rb

Lines 10 to 12 in 0059a0b

	def cask_installed?
	false
	end

It looks like Linux binaries are now allowed in casks (Homebrew/brew#19121), and are even being added to them in the official tap (Homebrew/homebrew-cask#199822). I'm not sure what logic would need to be changed but it looks like there would need to be some sort of Linux support check in https://github.com/Homebrew/homebrew-bundle/blob/master/lib/bundle/extend/os/linux/skipper.rb.