The ubuntu-20.04 runner is due to be deprecated on 2025-02-01 and
unsupported by 2025-04-01.
The ubuntu-24.04 runner doesn't include cbindgen so install it using
apt.
They're not reliably available and are now causing CI failures. Instead
use libloot for the test cases that used 7z, and update it because older
versions of libloot incorrectly use "ProductVERSION " instead of
"ProductVersion".
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.