stdenv: Turn on set -x if NIX_DEBUG >= 6

Why 6? It seems a decently high number, giving us room for more degrees
of debugging before the `set -x` sledgehammer without incurring a
mass-rebuild.

pkgs/stdenv/generic/setup.sh

@@ -1,6 +1,10 @@
 set -eu
 set -o pipefail
 
+if (( "${NIX_DEBUG:-0}" >= 6 )); then
+    set -x
+fi
+
 : ${outputs:=out}