Rollup merge of rust-lang#131176 - dev-ardi:gitignore, r=Noratrieb

.gitignore files for nix

This adds support for direnv and nix flakes / nix shell. I'm not sure if we should have a working nix configuration though.

r? ``@noratrieb``

Author: matthiaskrgr

.gitignore

@@ -86,4 +86,13 @@ package.json
 ## Rustdoc GUI tests
 tests/rustdoc-gui/src/**.lock
 
+## direnv
+.envrc
+.direnv/
+
+## nix
+flake.nix
+flake.lock
+default.nix
+
 # Before adding new lines, see the comment at the top.