fix(nixos): drop invalid --delete-generations flag from nix.gc.options

`--delete-generations` is a `nix-env` / `nix profile` flag; it's not
accepted by `nix-collect-garbage`, which only supports `--delete-old`
and `--delete-older-than`. PR #71 passed CI because flake-check only
evaluates the systemd unit — it never runs it — but `colmena apply`
aborts activation when `nix-gc.service` exits 1 with
"unrecognised flag '--delete-generations'".

Daily cadence + `--delete-older-than 7d` already covers both problems
#71 set out to fix: nixpkgs churn gets swept within 24h, and on servers
that finally redeploy after a long idle the previous-current generation
becomes eligible for pruning the next day.

Author: hakula139

modules/nixos/default.nix

@@ -93,7 +93,7 @@ in
       gc = {
         automatic = true;
         dates = "daily";
-        options = "--delete-older-than 7d --delete-generations +3";
+        options = "--delete-older-than 7d";
       };
       optimise.automatic = true;
     };