feat(nix-based): Better portability for sysexts

[tulilirockz]

Portability could be increased in this project in both of these cases:

Runtime

We could, instead of mounting /usr/store onto /nix/store, which requires root (currently, see #17), use nix-user-chroot or bubblewrap to wrap all the /usr/extension.d/$EXTENSION/bin symlinks to them and include /usr/store as /nix, making it think /nix/store exists when it doesnt, that would also remove the requirement for root for mounting /nix

Building

We could also build stuff with nix-portable or nix-user-chroot instead of spinning up a docker/podman container so that people may use bext layer build inside a distrobox or something like that