Force gem installation into GEM_HOME

When `--user-install` is set in the gemrc config file, `gem` ignores
`GEM_HOME`. `--no-user-install` prevents this behaviour.

pre_commit/languages/ruby.py

@@ -138,6 +138,7 @@ def install_environment(
                 (
                     'gem', 'install',
                     '--no-document', '--no-format-executable',
+                    '--no-user-install',
                     *prefix.star('.gem'), *additional_dependencies,
                 ),
             )