fix: use watch_file in .envrc (bazelbuild#2343)

`nix_direnv_watch_file` is deprecated and appears to have been removed
from some `direnv` distributions.

.envrc

@@ -6,5 +6,5 @@
 #  - https://marketplace.visualstudio.com/items?itemName=mkhl.direnv
 
 # Use `path:` syntax to avoid copying the entire repo to the Nix Store.
-nix_direnv_watch_file ./nix/flake.nix
+watch_file ./nix/flake.nix
 use flake path:./nix