Cross-build arm64 container #32
Description
The arm64 container currently builds in an emulated arm64 host via qemu-user. It would be much more efficient to cross-build from amd64 by having the Dockerfile specify FROM --platform=$BUILDPLATFORM for the builder container and set GOARCH=$TARGETARCH. However, Buildah < 1.24.1 doesn't support --platform in FROM. Once a new enough Buildah has landed in ubuntu-latest, switch to cross-building, and re-enable arm64 container builds in PRs by dropping the pr-arches override.
Followup to #31. See also coreos/butane#334.