stdenv: Make condition clearer

-z checks for a empty string
-n checks for a not empty string

It makes more sense to run the chmod if the string is not empty

pkgs/stdenv/generic/setup.sh

@@ -1594,7 +1594,7 @@ genericBuild() {
 
         if [ "$curPhase" = unpackPhase ]; then
             # make sure we can cd into the directory
-            [ -z "${sourceRoot}" ] || chmod +x "${sourceRoot}"
+            [ -n "${sourceRoot:-}" ] && chmod +x "${sourceRoot}"
 
             cd "${sourceRoot:-.}"
         fi