only ignore {flake,default}.nix and {.envrc,.direnv/} in the root

this makes it so files in `src/nix-dev-shell` are *not* ignored, as they
should not be. note that `flake.lock` is still ignored globally.

Author: WaffleLapkin

.gitignore

@@ -88,12 +88,12 @@ package.json
 tests/rustdoc-gui/src/**.lock
 
 ## direnv
-.envrc
-.direnv/
+/.envrc
+/.direnv/
 
 ## nix
-flake.nix
+/flake.nix
 flake.lock
-default.nix
+/default.nix
 
 # Before adding new lines, see the comment at the top.