[ci] Allow skipping builds with [skip build]

[asmaloney]

When working with the XML doc files, we don't need to build everything with every PR (as discussed with @YuriSizov in #64164).

With this change, adding the text [skip build] to a commit message will skip the builds and just run the static checks. (You should see the results with the GitHub actions on this PR -- hmmm I know there are issues trying to change CI Actions with PRs - here's my actions on this commit).

This will save a lot of time working on the docs and also stop a lot of extra builds. No more waiting for a macOS instance to become available!

[YuriSizov]

I think our preferred approach would be to chain CI tasks in a way where cheapest and quickest tasks run first, and if they fail, the rest doesn't run at all. Because we have quite a few issues with CI clogging on low-capacity platforms (macOS) or on heavy builds.

Making docs-only builds makes sense, but I think we should automate it based on the files changed, not on a commit tag. We don't want contributors to skip important checks by accident or out of malicious intent.

cc @akien-mga

[asmaloney]

We don't want contributors to skip important checks by accident or out of malicious intent.

Sure, right now they can [skip ci] to skip it all though. Same problem. All PRs are reviewed, so the reviewer will see that the build has been skipped.

Adding checks for specific changes is also possible, of course, but IMHO it will add complexity to the actions and the potential maintenance for it isn't worth it for this (niche) case. Happy to be wrong if there's now an easy way to do it.

I just want to stop building so much every single PR!

Edit: Example of how we might do it if only a specific folder changes. Ick 😬
Edit 2: A third-party action maybe?

[asmaloney]

I think we should automate it based on the files changed, not on a commit tag

Just to revive this since it's still an issue, maybe another possibility is to use paths-ignore (docs) in the workflows?

on:
  push:
    paths-ignore:
      - 'docs/**'

When all the path names match patterns in paths-ignore, the workflow will not run. If any path names do not match patterns in paths-ignore, even if some path names match the patterns, the workflow will run.

If we can enumerate the paths/files that should not trigger a build this might be a good solution?