Re-implement old mingw-w64 jobs in GHA. This allows to use the latest
Windows runners, replacing Windows Server 2012 R2 (gcc 6) and Windows
Server 2016 (gcc 7, 9) with Windows Server 2022.

GHA runners are also significantly faster, and allow running tests in
parallel (-j14). It also offloads 3 more long-running jobs from
AppVeyor CI.

These jobs download (then cache) the mingw-w64 packages from their
original location, which allows flexibility in choosing which versions
and flavours (win32/POSIX, SEH/DWARF, 64/32-bit) we want to test in CI.
The new jobs use these distros:

• https://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Personal%20Builds/mingw-builds/ (for gcc 7, same as on AppVeyor)
• https://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/ (for gcc 6, same as on AppVeyor)
• https://winlibs.com/ (for gcc 9)

I matched existing AppVeyor job configs, with these differences:

• gcc 6.4.0 instead of 6.3.0.
  (same distro as on AppVeyor, but the latest bugfix release)
• gcc 9.5.0 instead of 9.1.0 and a different (but compatible) binary distro.
  (in AppVeyor this relies on an old MSYS2 pre-installed on the runner)
• using win32 builds instead of posix for gcc 6.4.0 and 7.3.0.
• websockets enabled.
• always build examples.
• always build tests (this wasn't done for 6.4.0 with AppVeyor CI).

I did not replicate existing test exclusions, and oddly enough the few
failures (so far) were different from MSYS2 jobs and also from their
AppVeyor CI counterparts.

Also:

• delete redundant (default) -u option from cygpath calls.
• allow matrix options to override default ones in CMake.
• detect and use Windows-supplied curl for TFLAGS -ac option.
  (it's available in modern runners.)
• delete the 3 AppVeyor CI jobs now replicated in GHA.
• appveyor: prefer SYSTEMROOT over WINDIR.
• tidy-up quotes.

Job performance:

                                                                 AppVeyor  GHA
                                                                           w/examples
                                                                           w/tests
                                                                 --------  ----------
CMake, mingw-w64, gcc 6, Debug, x86, Schannel, Static, no-unity   1m25s     8m50s
CMake, mingw-w64, gcc 7, Debug, x64, Schannel, Static, Unicode   31m45s     9m39s
CMake, mingw-w64, gcc 9, Debug, x64, Schannel, Static            28m25s    13m38s

Based on these runs:
https://ci.appveyor.com/project/curlorg/curl/builds/49880799
https://github.com/curl/curl/actions/runs/9218292508

Notice that building examples and tests is time consuming.

We can tweak any build parameter as necessary to make them more useful
and/or without clogging the job queue or introducing flakiness.

Closes #13759

• make caching work. [Right, to make it work, path: must use Windows path notation. Not even ~ works, despite the documentation and apparently all cache examples are meant for *nix runners.]
• cleanup the PR, maybe move off some changes to a different PR?
• delete jobs from AppVeyor CI.
• saving the cache works now to a point, but then complains that: 'Failed to save: Unable to reserve cache with key Windows-mingw-w64-6.4.0-i686, another job may be creating this cache. More details: Cache already exists. Scope: refs/pull/13759/merge, Key: Windows-mingw-w64-6.4.0-i686, Version: 7029f2ef7decc13f34a275491ff299ebeebf54f184f7c19a012a4c41c363a793', then fails to find the cache with: 'Cache not found for input keys: Windows-mingw-w64-6.4.0-i686'. Perhaps this will be gone when merging this to master? Ignoring this for now, for better or worse. → It fixed itself after merging to master.