stdenv: avoid setuid issues

See #300635. Maybe in time we'll have a better solution.

pkgs/stdenv/generic/setup.sh

···
     # Make sure everything is writable so "strip" et al. work.
     local output
     for output in $(getAllOutputNames); do
-        if [ -e "${!output}" ]; then chmod -R u+w "${!output}"; fi
+        # for set*id bits see #300635
+        if [ -e "${!output}" ]; then chmod -R u+w,u-s,g-s "${!output}"; fi
     done
 
     runHook preFixup