formula: always override user home

Closes Homebrew/homebrew#40444.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/formula.rb

@@ -946,8 +946,17 @@ class Formula
   def stage
     active_spec.stage do
       @buildpath = Pathname.pwd
-      yield
-      @buildpath = nil
+      env_home = buildpath/".brew_home"
+      mkdir_p env_home
+
+      old_home, ENV["HOME"] = ENV["HOME"], env_home
+
+      begin
+        yield
+      ensure
+        @buildpath = nil
+        ENV["HOME"] = old_home
+      end
     end
   end