Nix.pm: readNixFile: pass «--experimental-features nix-command»

Declarative jobsets were broken by the Nix update, causing
nix cat-file to break silently.

This commit restores declarative jobsets, based on top of a commit
making it easier to see what broke.

src/lib/Hydra/Helper/Nix.pm

@@ -509,7 +509,8 @@ sub getStoreUri {
 # Read a file from the (possibly remote) nix store
 sub readNixFile {
     my ($path) = @_;
-    return grab(cmd => ["nix", "cat-store", "--store", getStoreUri(), "$path"]);
+    return grab(cmd => ["nix", "--experimental-features", "nix-command",
+                        "cat-store", "--store", getStoreUri(), "$path"]);
 }