fix: compatibility with latest WSL update

[K900]

The latest WSL update makes /dev/shm a mount point, not a symlink, as it should be.
Unfortunately, this does not work well with us trying to remove it. Oops.

[K900]

A quick "my system is broken pls help" guide:

1. Check out this PR
2. nix build .#staticShim
3. Copy result/bin/nixos-systemd-shim somewhere on the Windows file system
4. From a Windows shell, run wsl --system --user=root
5. mount -o remount,rw /mnt/wslg/distro
6. rm /mnt/wslg/distro/sbin/init
7. cp /mnt/.../nixos-systemd-shim /mnt/wslg/distro/sbin/init
8. Exit WSL and run wsl --shutdown
9. Restart NixOS on WSL
10. Rebuild your config with changes from this PR - otherwise the activation script will replace the shim with the old broken one.

[nzbr]

Code looks good so far. Will this work on the older WSL version as well?

[K900]

It should.

[SuperSandro2000]

Tested on the current stable WSL release and thing still work after a wsl --shutdown