Move to new CI (GitHub Actions?)

[workingjubilee]

QEMU sometimes throws apparently spurious errors, and in any case adds a slowdown. It would be nice to take advantage of the fact that Travis offers the following arch tags:

arch: arm64
arch: ppc64le
arch: s390x

and actually run on those architectures for the relevant tests.

[workingjubilee]

Travis CI has become totally unsuitable.