Enable direnv (#31672) (#31679)

Backport #31672 by @techknowlogick

This lets developers who have direnv enabled to load our nix flake
automatically when entering it

Co-authored-by: techknowlogick <techknowlogick@gitea.com>

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -108,6 +108,9 @@ prime/
 *_source.tar.bz2
 .DS_Store
 
+# nix-direnv generated files
+.direnv/
+
 # Make evidence files
 /.make_evidence