Experiemtal and impure are now requires!

Co-authored-by: Florian Sesser <florian@leastauthority.com>
Signed-off-by: Benoit Donneaux <benoit@leastauthority.com>

nixos/modules/update-deployment

@@ -82,7 +82,7 @@ ssh -o StrictHostKeyChecking=no "$(hostname).$(domainname)" ":"
 
 # Two lines since 'export' masks 'set -e'
 # See https://mywiki.wooledge.org/BashFAQ/105#line-204
-NIX_PATH="nixpkgs=$(nix eval "(import ${CHECKOUT}/nixpkgs.nix { }).path")"
+NIX_PATH="nixpkgs=$(nix --extra-experimental-features nix-command eval --impure --expr "(import ${CHECKOUT}/nixpkgs.nix { }).path")"
 export NIX_PATH
 
 # Attempt to update just this host.  Choose the morph grid definition matching