This fixes slim-lint not honoring a `.rubocop.yml` in the file's or
parent directory. Due to the way slim-lint calls rubocop, it requires
the special `SLIM_LINT_RUBUCOP_CONF` env var to pick up the
`.rubocop.yml` if it is not run on the real file (which is the case
here).
See https://github.com/sds/slim-lint/blob/master/lib/slim_lint/linter/README.md#rubocop
