stdenv/multiple-outputs: Fix checking of propagatedOutputs

'[[ ! -v "$propagatedOutputs" ]]' is incorrect and always evaluates to
true. The correct form using double brackets would be
'[[ ! -v propagatedOutputs ]]', but I strongly dislike '[[ ]]' due to
the totally different quoting rules compared to everything else in bash.

pkgs/build-support/setup-hooks/multiple-outputs.sh

@@ -136,7 +136,7 @@ _multioutPropagateDev() {
     done
 
     # Default value: propagate binaries, includes and libraries
-    if [[ ! -v "$propagatedOutputs" ]]; then
+    if [ -z "${propagatedOutputs+1}" ]; then
         local po_dirty="$outputBin $outputInclude $outputLib"
         set +o pipefail
         propagatedOutputs=`echo "$po_dirty" \