Add build job with SFML_ENABLE_PCH=1

[vittorioromeo]

Useful to catch regressions with PCH support enabled.

[eXpl0it3r]

I guess the question is, whether we should expand the buildbot matrix or expand the GitHub Actions matrix.

What do you think, @ChrisThrasher?

[ChrisThrasher]

I'd rather do more work in GitHub Actions since it's tied to version control. I'd prefer if the Buildbot was reserved for things that are unfeasible in GitHub Actions like building on FreeBSD or with MinGW.

[binary1248]

As @ChrisThrasher said, buildbot is preferred when we are building for exotic configurations. If something can be tested on the VMs that actions provides it should be preferred instead.