fix: switch CRN droplets to ubuntu-24.04, DO retired debian-12-x64 (#33)

DigitalOcean no longer offers the debian-12-x64 image (only debian-13-x64
remains), so PR CI fails at droplet creation with 422 'invalid image'.

Move the DO CRN droplets to ubuntu-24-04-x64, the same distro as the CCN
droplet. This keeps the sevctl binary copied from CRN 0 guaranteed to run
on the CCN (debian-13's glibc 2.41 is newer than ubuntu-24.04's 2.39 and
would break that).

Also make the branch (CI artifact) install path pick the aleph-vm .deb per
CRN distro via /etc/os-release, like the release path already does: aleph-vm
CI builds all four distro debs, and a static TEE server may not run the same
distro as the DO droplets.