direnv: Watch changes to default.nix

Now that the package definition lives in default.nix we need direnv to
watch that file to get automatic reloads.

.envrc

@@ -1,7 +1,8 @@
 watch_file shell.nix
+watch_file default.nix
 watch_file flake.lock
 watch_file rust-toolchain.toml
 
 # try to use flakes, if it fails use normal nix (ie. shell.nix)
 use flake || use nix
-eval "$shellHook"
+eval "$shellHook"