v0.7.3

rename rocky to rockylinux; new default images for netbsd, midnightbsd
and openeuler

Rocky Linux is now --os rockylinux, the same name as its builder
(anyvm-org/rockylinux-builder, renamed from rocky-builder) and its
GitHub Action (vmactions/rockylinux-vm). The default image is
rockylinux-builder v2.0.1. The old name is not kept as an alias, so
replace --os rocky with --os rockylinux: anyvm no longer applies the
Rocky Linux settings (virtio NIC, TCG under nested Windows) to "rocky".

New default builder versions:

netbsd 2.2.6 -> 2.2.7 adds NetBSD 10.2, fixes 9.x packages
midnightbsd 2.0.7 -> 2.0.8 MidnightBSD 4.0.8
openeuler 2.0.5 -> 2.0.6 adds openEuler 26.09 (x86_64, aarch64)
rockylinux 2.0.1 was rocky 2.0.0

Fixed: Redox no longer fails to mount its root filesystem when the host
is slow to sync the disk image. Redox 0.9.0's IDE driver gives each ATA
command 1 s and flushes after every write, while QEMU completes a guest
flush only once the host fdatasync returns. The Redox disk now runs with
cache.no-flush=on; guest writes still reach the host page cache, so
only a host crash could lose them.

Other: --debug now logs the host CPU model on every run, not only when
a boot times out. CI tests NetBSD 10.2, and the builder bump bot also
picks up point releases of older branches.

Full Changelog: v0.7.2...v0.7.3