common setup hooks: set -u robustness

Explicitly handle `dont*` and friends not being defined.

pkgs/build-support/setup-hooks/move-sbin.sh

@@ -5,7 +5,7 @@
 fixupOutputHooks+=(_moveSbin)
 
 _moveSbin() {
-    if [ "$dontMoveSbin" = 1 ]; then return; fi
+    if [ "${dontMoveSbin-}" = 1 ]; then return; fi
     if [ ! -e "$prefix/sbin" -o -L "$prefix/sbin" ]; then return; fi
     echo "moving $prefix/sbin/* to $prefix/bin"
     mkdir -p $prefix/bin