Fix HEAD installations with HOMEBREW_FORBID_PACKAGES_FROM_PATHS

- Allow cache paths in FromPathLoader when HOMEBREW_FORBID_PACKAGES_FROM_PATHS is set
- Fixes issue where HEAD installations fail due to temporary source downloads
- Add test case to verify cache paths are allowed when path restrictions are enabled

The issue occurred because HEAD installations download formula sources to cache
directories, but HOMEBREW_FORBID_PACKAGES_FROM_PATHS only allowed paths from
HOMEBREW_CELLAR and HOMEBREW_LIBRARY/Taps, causing the installation to fail.

Closes: homebrew/brew#issue-number

Library/Homebrew/formulary.rb

@@ -617,7 +617,8 @@ module Formulary
       return unless path.expand_path.exist?
 
       return if Homebrew::EnvConfig.forbid_packages_from_paths? &&
-                !path.realpath.to_s.start_with?("#{HOMEBREW_CELLAR}/", "#{HOMEBREW_LIBRARY}/Taps/")
+                !path.realpath.to_s.start_with?("#{HOMEBREW_CELLAR}/", "#{HOMEBREW_LIBRARY}/Taps/",
+                                                "#{HOMEBREW_CACHE}/")
 
       if (tap = Tap.from_path(path))
         # Only treat symlinks in taps as aliases.

Library/Homebrew/test/formulary_spec.rb

@@ -141,6 +141,21 @@ RSpec.describe Formulary do
         end.to raise_error(FormulaUnavailableError)
       end
 
+      it "allows cache paths even when paths are disabled" do
+        ENV["HOMEBREW_FORBID_PACKAGES_FROM_PATHS"] = "1"
+        cache_dir = HOMEBREW_CACHE/"test_formula_cache"
+        cache_dir.mkpath
+        cache_formula_path = cache_dir/formula_path.basename
+        FileUtils.cp formula_path, cache_formula_path
+        begin
+          formula = described_class.factory(cache_formula_path)
+          expect(formula).to be_a(Formula)
+        ensure
+          cache_formula_path.unlink if cache_formula_path.exist?
+          cache_dir.rmdir if cache_dir.exist?
+        end
+      end
+
       context "when given a bottle" do
         subject(:formula) { described_class.factory(bottle) }