template: use direnv only

nix-direnv depends on NIX_BUILD_TOP, which is removed from our cleaned
environment.

Fixes #188

Author: zimbatm

template/.envrc

@@ -1,5 +1,7 @@
-if ! has nix_direnv_version || ! nix_direnv_version 1.3.0; then
-  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/1.3.0/direnvrc" "sha256-1VWM1BnI1GvclYBky5f5Y9HqeThmQUwCWQbsFQM1Eu0="
+if [[ $(type -t use_flake) != function ]]; then
+  echo "ERROR: use_flake function missing."
+  echo "Please update direnv to v2.30.0 or later."
+  exit 1
 fi
 
 use flake