Clarify .github/scripts/generate_ci_workflows.py (#58498)
Summary:
Followup to https://github.com/pytorch/pytorch/issues/58491:
- use f-string to remove the literal `generated` string from the generator script, so Phabricator no longer thinks it is a generated file
- remove the special logic for `test_runner_type` and instead explicitly specify for every workflow
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58498
Test Plan:
```
make generate-gha-workflows
```
Also, check that Phabricator doesn't classify `.github/scripts/generate_ci_workflows.py` as "Generated changes" in this diff.
Reviewed By: seemethere
Differential Revision: D28516291
Pulled By: samestep
fbshipit-source-id: 8736eaad5d28082490be0a9b2e271c9493c2ba9d
