change ensureDir to mkdir -p in example

ensureDir was dropped with the merge of https://github.com/NixOS/nixpkgs/pull/33637

pills/20/propagated-build-inputs-0.bash

@@ -3,7 +3,7 @@ fixupPhase() {
     ## Elided
 
     if test -n "$propagatedBuildInputs"; then
-        ensureDir "$out/nix-support"
+        mkdir -p "$out/nix-support"
         echo "$propagatedBuildInputs" > "$out/nix-support/propagated-build-inputs"
     fi