Rely on cf-built qemu?

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

https://github.com/conda-forge/docker-images/blob/a13e64f37972bfa1ad2cc21335f0944b427d1526/download-qemu-static.sh#L17-L36

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