There's nothing wrong with Travis CI, but AppVeyor has started to fail
builds with a toolchain error when they install rustfmt. This is the
second time some kind of toolchain change has broken my builds, so I'm
migrating away from it.
I could have migrated to Travis CI's relatively new Windows support, but
that's still in the early adoption phase according to its own docs, and
I might as well instead move to GitHub's first-party offering. There's
no point then using Travis CI for Linux builds when that can be done on
GitHub Actions too.
