Lint: move "lint-strict" target to nightly schedule

Since my 688682aa (Lint: add a "lint-strict" CI target, 2021-01-06) we've been showing the (!) checkmark next to all MRs instead of the green/red OK/fail checkmark.

Since these strict lint failures are just something we want to keep an eye on and not really actionable for any one MR author let's move them to the nightly run accessible at https://gitlab.com/gitlab-org/gitaly/-/pipeline_schedules