build: add nodejs dep to flake env

This was always required, but was omitted in the flake env, causing
`node` from system PATH to be used. It's more explicit to track a
compatible version, so that `just dev` works out of the box for nix
users.

flake.nix

@@ -18,6 +18,7 @@
             buildInputs = [ 
                 firebase-tools
                 just
+                nodejs_20
                 pnpm
             ];
           };