ci: make sure directories are created only under dir_prefix

ci: fix breakage

lzma: update to 21.07

xz-embedded: update

ci: move workflow to the right place

Dockerfile: add x86_64-any64-linux-musl toolchain

ci: build container on GitHub Actions

ci: add x86_64 binaries

ci: upload artifacts to a GitHub release

minlzma: use upstream