Fix deployment script: Add experimental nix-command feature (and explicitly allow impurity)

ci-tools/update-grid-servers

@@ -79,7 +79,7 @@ update_grid_nodes() {
     # Find the names of all hosts that belong to this grid.  This list includes
     # one extra string, "network", which is morph configuration stuff and we need
     # to filter out later.
-    nodes=$(nix eval --json "(builtins.concatStringsSep \" \" (builtins.attrNames (import $grid_dir/grid.nix)))" | jp --unquoted @)
+    nodes=$(nix --extra-experimental-features nix-command eval --impure --json "(builtins.concatStringsSep \" \" (builtins.attrNames (import $grid_dir/grid.nix)))" | jp --unquoted @)
 
     # Tell every server in the network to update itself.
     for node in ${nodes}; do