Only produce .xz tarballs on CI
This PR adds a `./configure` option to choose which tarball compression formats to produce, and changes our CI configuration to only produce `.xz` tarballs. The release process will then recompress everything into `.gz` when producing a release.
This will drastically reduce our storage costs for CI artifacts, as we'd stop storing the same data twice. **Stable, beta and nightly releases will not be affected by this at all.**
Before landing this we'll need to increase the VM size of our release process, to recompress everything in a reasonable amount of time.
r? `@Mark-Simulacrum`
f0073a59cf