gitlab-ci: Tag linter stage
gitlab-ci: Tag linter stage

Previously the linters were tagged with x86_64-linux, meaning that linting jobs
would often get stuck behind builds in the queue. Given that linting jobs are
fairly low-cost they hold up later build stages it is important that we reduce
this latency.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc