Rely on cf-built qemu?

[h-vetinari]

We have https://github.com/conda-forge/qemu-execve-feedstock now, and assuming this works, I think it'd be much nicer to rely on a conda-forge package than

current code to vendor qemu

docker-images/download-qemu-static.sh

Lines 17 to 36 in a13e64f

	# see https://gitlab.com/qemu-project/qemu/-/tags for versions;
	# we use the RPMs from https://kojipkgs.fedoraproject.org/packages/qemu;
	# avoid qemu builds from unreleased fedora versions, compare `build`
	# vs. https://en.wikipedia.org/wiki/Fedora_Linux_release_history;
	# prefer non-`.0` patch releases to try to avoid potential new regressions;
	# if possible, check https://gitlab.com/qemu-project/qemu/-/issues
	# for relevant issues in old vs new version;
	version='8.2.8'
	build='2.fc40'
	for arch in aarch64 ppc64le s390x; do
	curl -sL \
	"https://kojipkgs.fedoraproject.org/packages/qemu/${version}/${build}/x86_64/qemu-user-static-${arch/ppc64le/ppc}-${version}-${build}.x86_64.rpm" |
	bsdtar -xf- --strip-components=3 ./usr/bin/qemu-${arch}-static
	done
	sha256sum --check << 'EOF'
	c41cd478bdcccbc76a0e35db8ba65861038cd8f0d6339abc0cfd19eadc335fc6 qemu-aarch64-static
	9b5c44f35eceaf6484ec11bc03047001293586f9ae73861dde87329243d56ae7 qemu-ppc64le-static
	767a23c0ec4570b28d352ad00c55c4fc2315d5707078d022c1d2cc07d827561e qemu-s390x-static
	EOF

We also had issues with a major regression in QEMU v9 that forced us to go back to v8 (xref #300). It's very cumbersome to test these sort of changes before deploying them (we'd have to suppress that the :latest tag gets updated, and then override the image+tag in affected feedstocks). It'd be much nicer if we could just dial in the qemu version in remote_ci_setup via smithy, so that feedstocks can override this version locally without too much hassle.

Thoughts @conda-forge/core?

CC @conda-forge/qemu-execve

This would obsolete #298