[lint] Move shellcheck to its own step (#58623)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58623
This splits out everything shellcheck related into its own job that generates and checks GHA workflows, then shellchecks those + jenkins scripts. This PR also integrates shellcheck into the changed-only stuff in `actions_local_runner.py` so that shellcheck won't do anything unless someone edits a shell script in their local checkout. This is the final piece to clean up the output of `make quicklint` and speeds it up by a good bit (before it was shellchecking everything which took a few seconds):
```
$ make quicklint -j $(nproc)
✓ quick-checks: Ensure no unqualified noqa
✓ quick-checks: Ensure canonical include
✓ quick-checks: Ensure no unqualified type ignore
✓ quick-checks: Ensure no direct cub include
✓ quick-checks: Ensure no tabs
✓ quick-checks: Ensure no non-breaking spaces
✓ shellcheck: Regenerate workflows
✓ quick-checks: Ensure no versionless Python shebangs
✓ quick-checks: Ensure correct trailing newlines
✓ shellcheck: Assert that regenerating the workflows didn't change them
✓ mypy (skipped typestub generation)
✓ cmakelint: Run cmakelint
✓ quick-checks: Ensure no trailing spaces
✓ flake8
✓ shellcheck: Extract scripts from GitHub Actions workflows
✓ shellcheck: Run Shellcheck
real 0.92
user 6.12
sys 2.45
```
Test Plan: Imported from OSS
Reviewed By: nikithamalgifb
Differential Revision: D28617293
Pulled By: driazati
fbshipit-source-id: af960ed441db797d07697bfb8292aff5010ca45b
