cmd/fetch & cmd/audit: handle unsupported os/arch combos

It's possible for casks to only run on certain os/arch
combinations but we don't check for them in the fetch and
audit commands.

At first, I tried to check if the macos version was satisfied
in a previous PR but that doesn't work correctly because
MacOSRequirement and ArchRequirement don't respect SimulateSystem.

Instead I'm just checking to see if the url stanza is defined
for each os/arch combination and skipping those casks where
it ends up being nil. This is kind of brute forcing it but
should work.

Library/Homebrew/cmd/fetch.rb

@@ -166,8 +166,8 @@ module Homebrew
           SimulateSystem.with os: os, arch: arch do
             cask = Cask::CaskLoader.load(ref)
 
-            if cask.depends_on.macos&.satisfied? == false
-              opoo "#{cask.token}: #{cask.depends_on.macos.message(type: :cask)}"
+            if cask.url.nil?
+              opoo "Cask #{cask} is not supported on os #{os} and arch #{arch}"
               next
             end
 

Library/Homebrew/dev-cmd/audit.rb

@@ -242,10 +242,17 @@ module Homebrew
         next [] if os == :linux
 
         SimulateSystem.with os: os, arch: arch do
+          simulated_cask = Cask::CaskLoader.load(path)
+
+          if simulated_cask.url.nil?
+            opoo "Cask #{cask} is not supported on os #{os} and arch #{arch}"
+            next []
+          end
+
           odebug "Auditing Cask #{cask} on os #{os} and arch #{arch}"
 
           Cask::Auditor.audit(
-            Cask::CaskLoader.load(path),
+            simulated_cask,
             # For switches, we add `|| nil` so that `nil` will be passed
             # instead of `false` if they aren't set.
             # This way, we can distinguish between "not set" and "set to false".