sysext-bakery: Add build helper based on e.g. Nix for distro-independent systemd-sysext images

[pothos]

Current situation

Currently the sysext-bakery scripts use static binaries to be version-independent instead of being coupled to a specific OS version.

Impact

This limits the software we can provide.

Ideal future situation

Add build helper based on e.g. Nix is added as alternative to the Flatcar SDK Gentoo prefix build option.

Even nicer if it would support arbitrary software trees from any distro.

Implementation options

To support arbitrary distros some ELF loader path patching could be tried to have the effect of LD_LIBRARY_PATH.

Additional information

Nix is a very common tool for self-contained software trees, normally people use it in their home dirs. But of course it's also used to build container images or as package manager in NixOS.

[pothos]

Here a set of tools to build distro-indep. sysext from a package-manager chroot (e.g. container image): flatcar/sysext-bakery#74

Tested with Debian and Alpine.
Didn't try the flix.sh with nix but for nix one might even make it simpler assuming that the elf-patching can be skipped.