Move AppVeyor CygWin and MinGW builds to GitHub Actions

[claremacrae]

Appveyor builds finish long after GitHub Actions builds.

We only build CygWin and MinGW on AppVeyor, so I am moving those builds to GitHub Actions instead.

[claremacrae]

• 2e2fb96bc5ed880a6ef67ccdafbd5cfdc8a3879d added Cygwin to GitHub Actions
• ba04539eacc137a6688772e36ee270787b8bba53 added MinGW to GitHub Actions
• 4e97869174572694ef4e66951ee57d309d2a7e33 disabled AppVeyor builds
• 3ea2d75358c31873e43a19f524fab170643ab120 removed the AppVeyor build badge

[claremacrae]

This is now done