envrc: use nix, nix flakes messes up env in a way that breaks shells

.envrc

@@ -1,6 +1,5 @@
 # If we are a computer with nix-shell available, then use that to setup
 # the build environment with exactly what we need.
 if has nix; then
-  # Can add ". --impure" if we ever need the parent shell environment. We might.
-  use flake
+  use nix
 fi