Version the workflows

[award999]

Even if we put #73 in place, we could tag or create release branches so that downstream "uses" of swift_package_test.yml specify a constant semvar version instead of always working off of main which is more susceptible to breaking.

For example: swiftlang/github-workflows/.github/workflows/[email protected]

Looks possible to automate with certain marketplace actions like https://github.com/marketplace/actions/tag-release-on-push-action, but manually bumping as need may be sufficient

[FranzBusch]

Yes we should tag releases at some point and depend on this. We still have some ongoing discussion if we can merge the NIO test workflows into this repo which might impact the API of the workflows. Once we figured that out we should move to Semantic versioning.

[MahdiBM]

To add to this, the downstream repos can use dependabot for GitHub Actions action-updates, so dependabot automatically prompts those repos to update the workflow versions, specially if there is a new major version, and also considering something like @v1 means "up to next major, from v1", and doesn't mean "pinned to v1".