cask/installer: improve *_FORBIDDEN_* env handling.

- Improve the error message when a cask or formula is forbidden by an
  environment variable (fixes #17880)
- Move the `forbidden_tap_check` and `forbidden_cask_and_formula_check`
  methods to the top of the `install` method, so that they are checked
  before the main cask is downloaded.

Library/Homebrew/cask/installer.rb

@@ -68,12 +68,12 @@ module Cask
       verify_has_sha if require_sha? && !force?
       check_requirements
 
+      forbidden_tap_check
+      forbidden_cask_and_formula_check
+
       download(quiet:, timeout:)
 
       satisfy_cask_and_formula_dependencies
-
-      forbidden_tap_check
-      forbidden_cask_and_formula_check
     end
 
     def stage
@@ -660,15 +660,15 @@ on_request: true)
           next if dep_name.blank?
 
           raise CaskCannotBeInstalledError.new(@cask, <<~EOS
-            The installation of #{@cask} has a dependency #{dep_name}
+            has a dependency #{dep_name} but the
-            but the #{dep_name} #{dep_type} was forbidden by #{owner} in `#{variable}`.#{owner_contact}
+            #{dep_name} #{dep_type} was forbidden for installation by #{owner} in `#{variable}`.#{owner_contact}
           EOS
           )
         end
       end
       return if forbidden_casks.blank?
 
-      cask_name = if forbidden_casks.include?(@cask.token)
+      if forbidden_casks.include?(@cask.token)
         @cask.token
       elsif forbidden_casks.include?(@cask.full_name)
         @cask.full_name
@@ -677,8 +677,7 @@ on_request: true)
       end
 
       raise CaskCannotBeInstalledError.new(@cask, <<~EOS
-        The installation of #{cask_name} was forbidden by #{owner}
+        forbidden for installation by #{owner} in `HOMEBREW_FORBIDDEN_CASKS`.#{owner_contact}
-        in `HOMEBREW_FORBIDDEN_CASKS`.#{owner_contact}
       EOS
       )
     end

Library/Homebrew/test/cask/installer_spec.rb

@@ -399,7 +399,7 @@ RSpec.describe Cask::Installer, :cask do
 
       expect do
         described_class.new(cask).forbidden_cask_and_formula_check
-      end.to raise_error(Cask::CaskCannotBeInstalledError, /#{cask_name} was forbidden/)
+      end.to raise_error(Cask::CaskCannotBeInstalledError, /forbidden for installation/)
     end
 
     it "raises on forbidden dependency" do