Nightly Windows builds are available again

[Benjamin Gilbert]

Hi all,

I'm pleased to announce an update on the OpenSlide Windows builds:

- Builds have been fixed to work correctly with a current MinGW-w64 toolchain.
- All dependencies have been updated.
- The build now runs in a Fedora container image that's refreshed
weekly by GitHub Actions, removing the need to manually install
dependencies on the build machine.  Container-based builds should work
both on Linux and on Windows with WSL2.
- Support for building in Cygwin has been removed.
- Nightly builds have been migrated to GitHub Actions and are running again.

The nightly builds are available at:

    https://openslide.org/builds/windows/

Please try them out and report problems.  If no issues are found, I
plan to release a new official Windows build in the coming weeks.
(Unlike the nightly builds, the official release will still include
OpenSlide 3.4.1, not the Git main branch.)

Best,
--Benjamin Gilbert