Support using GAP releases instead of branch snapshots

[fingolfin]

Right now, we install GAP always from git; to test against e.g. 4.10, we use the stable-4.10 branch.

But it would be useful to (also?) be able to test against actual releases, such as 4.10.2 or 4.9.3. One important difference is that GAPInfo.Version is set correctly there (in git build, it is always 4.dev or something like that, which means version checks in code don't work).

Ideally, the user would be able to either specify "4.10" (and then gets whatever the latest 4.10.x is) or even a specific version like "4.10.2" (however, this later point is less important).

[fingolfin]

Doing this shouldn't be too hard either:

• add an alternate input to GAPBRANCH, say version
• accept values of the form X.Y or X.Y.Z for them
• the former (version: X.Y) is automatically mapped to version: X.Y.Z the version of the latest release of the X.Y release series
  • for this we can just hardcode a list in this repo, and update the repo / this action when there are new releases
• version: latest would use the latest stable release
• version: BRANCHNAME to test against that GAP branch
  • we'd limit this to stable-* and master for now
  • in addition we could also support a format like ORG/REPO#branch to permit using a branch from an arbitrary GAP fork
• the Clone GAP step then is changed to say Get GAP sources and we download GAP from the appropriate place (i.e., for all new enough releases from https://github.com/gap-system/gap/releases/download/vX.Y.Z/gap-X.Y.Z.tar.gz -- actually, we could retroactively provide these for 4.9.x and 4.10.x, too (I don't think it makes sense to try to support older GAP versions for this)

That should be most of it.

Of course if we were to switch to docker images (see also issue #16) this may become moot, but unless someone actively works on that, I don't see why we shouldn't try the above (or a variation thereof)