nixosTests.initrd-luks-empty-passphrase: mount the host nix store

This is necessary because this test relies on switching the root fs to an empty one which
does not have a Nix store available in stage 1, therefore, we have to make this test
host-store only.

A better fix in the long term is to evaluate whether this is worth to enable a proper
Nix store image for it with EROFS?

nixos/tests/initrd-luks-empty-passphrase.nix

···
       emptyDiskImages = [ 512 ];
       useBootLoader = true;
       useEFIBoot = true;
+      # This requires to have access
+      # to a host Nix store as
+      # the new root device is /dev/vdb
+      # an empty 512MiB drive, containing no Nix store.
+      mountHostNixStore = true;
     };
 
     boot.loader.systemd-boot.enable = true;