sbt-github-actions: run githubWorkflowGenerate after build.sbt changes

[kubukoz]

Follow-up to #1745.

Some changes in build.sbt may cause a change to the workflow files, e.g. a Scala version upgrade.

In these situations, Steward could regenerate the YAML configs as well. The exact keys that can influence that result might be hard to detect, so I guess it could run automatically on every change to .sbt / .scala build files if there's sbt-github-actions in the build.

[fthomas]

I like this but would restrict it to Scala updates. Nearly all Scala Steward PRs change .sbt or .scala files which means we would need to run sbt githubWorkflowGenerate on all updates if sbt-github-actions is used. That could slow down Scala Steward considerably because running sbt is slow.

[kubukoz]

right, if we can't reuse the previous session with a reload it probably doesn't make sense to run every time. 👍

[fthomas]

We do not run sbt every time a PR is created, so in most cases there is just no previous sbt session we could reuse.