nixosTests.systemd-initrd-btrfs-raid: mount the host Nix store

Same rationale as caf6f41e2e1.

nixos/tests/systemd-initrd-btrfs-raid.nix

···
     virtualisation = {
       emptyDiskImages = [ 512 512 ];
       useBootLoader = true;
+      # Booting off the BTRFS RAID requires an available init script from the Nix store
+      mountHostNixStore = true;
       useEFIBoot = true;
     };
     boot.loader.systemd-boot.enable = true;