The point of this build is to ensure the project is able to build with
the toolchain available in debian stable (currently Go 1.24). Without
GOTOOLCHAIN=local, `go build` will automatically download a newer
toolchain if required, defeating the purpose of the test.
Switch from Travis and AppVeyor to Github Actions.
We keep running out of OSS credits from Travis, and it's a pain having to request more credits via email every time.