* flake: format
* image: update mkosi to 24.3
This updates mkosi to a next-version of v24.3, which is now available in nixpkgs. This removes the non-hermetic `uidmap` dependency, which is a great advantage. It will also be less of an effort to upgrade to v25 going forward.
Changes required are keeping `/var/cache` around (which is reproducible for our images, so no problem), as mkosi needs files from it in the build process. mkosi now additionally requires an explicit option to fetch the signing keys for the package repositories from the internet. A hack was required to satisfy the Bazel package, which should probably be solved properly at some point.
This allows cross compiling from aarch64-darwin to x86_64-linux.
It is required for building Go binaries on macos that target Linux and have CGO enabled.
