utils/gems: handle mismatching EUID and UID for bundle installs

Library/Homebrew/dev-cmd/install-bundler-gems.rb

@@ -32,7 +32,6 @@ module Homebrew
           Homebrew.forget_user_gem_groups!
         end
 
-        Process::UID.change_privilege(Process.euid) if Process.euid != Process.uid
         Homebrew.install_bundler_gems!(groups:)
       end
     end

Library/Homebrew/utils/gems.rb

@@ -300,7 +300,12 @@ module Homebrew
       end
 
       bundle_installed = if bundle_install_required
-        if system bundle, "install", out: :err
+        Process.wait(fork do
+          # Native build scripts fail if EUID != UID
+          Process::UID.change_privilege(Process.euid) if Process.euid != Process.uid
+          exec bundle, "install", out: :err
+        end)
+        if $CHILD_STATUS.success?
           true
         else
           message = <<~EOS