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]

• 2e2fb96 added Cygwin to GitHub Actions
• ba04539 added MinGW to GitHub Actions
• 4e97869 disabled AppVeyor builds
• 3ea2d75 removed the AppVeyor build badge

[claremacrae]

This is now done