nightly-v7.0.1

Enhance 'make dist', add GitHub release workflow

- a new file VERSION is the only place storing the FLINT version
- generate src/flint.h from src/flint.h.in so we can insert the version
- revise `make dist` to call a helper script `dev/make_dist.sh`, which
  creates all relevant archives, which include files generated by autoconf
- add GitHub Workflows to tests creation and usability of these archives
  on various conditions