flake: upport channel-less in NUR check

Interestingly doesn't actually fail like `nix flake check` does.

flake.nix

@@ -37,11 +37,16 @@
             # Normally we'd also run with `--option restrict-eval true`, but
             # this is incompatible with flakes because reasons.
             nur = pkgs.writeShellScriptBin "nur-check" ''
+              # Prefer nixpkgs channel (actual build), otherwise read from flake.lock (CI)
+              if ! nixpkgs=$(nix-instantiate --find-file nixpkgs 2>/dev/null); then
+                _rev=$(${pkgs.jq}/bin/jq -r .nodes.nixpkgs.locked.rev flake.lock)
+                nixpkgs="https://github.com/nixos/nixpkgs/archive/''${_rev}.tar.gz"
+              fi
               nix-env -f . -qa \* --meta \
                 --allowed-uris https://static.rust-lang.org \
                 --option allow-import-from-derivation true \
                 --drv-path --show-trace \
-                -I nixpkgs=$(nix-instantiate --find-file nixpkgs) \
+                -I nixpkgs="$nixpkgs" \
                 -I ./ \
                 --json | ${pkgs.jq}/bin/jq -r 'values | .[].name'
             '';