waf: move binary to $out/bin/

Originally this was just the executable, but this breaks nix-shell
usage. Instead, just do a normal directory.

Fixes #79909

pkgs/development/tools/build-managers/wafHook/setup-hook.sh

@@ -3,7 +3,7 @@ wafConfigurePhase() {
 
     if ! [ -f "${wafPath:=./waf}" ]; then
         echo "copying waf to $wafPath..."
-        cp @waf@ "$wafPath"
+        cp @waf@/bin/waf "$wafPath"
     fi
 
     if [ -z "${dontAddPrefix:-}" ] && [ -n "$prefix" ]; then