.github/workflows/dist.yml: Create github release on tag

[mkoeppe]

This new GitHub Actions workflows makes it easier to cut a release.

Just push a release tag (v2.*) to the repository, and it will run make distcheck to build a release tarball, create a GitHub release, and upload the tarball to it.

Included is a fix for make distclean.

The workflow uses upstream libtool 2.4.7 instead of the heavily patched version provided on Ubuntu. Generating the release tarball like this fixes a linking problem on macOS, see sagemath/sage#38020 (comment)

Preview of the workflow run: https://github.com/mkoeppe/fflas-ffpack/actions/runs/9141750673/job/25136537002
The generated release: https://github.com/mkoeppe/fflas-ffpack/releases/tag/v2.5.0%2Bsage-2024-05-18b

[fingolfin]

That sounds like a very nice idea, esp. if it would help get out a new release of this package and other under the linbox-team umbrella.