root: fix setupHook to work with "set -u"

pkgs/applications/science/misc/root/setup-hook.sh

@@ -1,9 +1,13 @@
 thisroot () {
     # Workaround thisroot.sh dependency on man
-    if [ -z "${MANPATH}" ]; then
+    if [ -z "${MANPATH-}" ]; then
         MANPATH=:
     fi
+    local oldOpts="-u"
+    shopt -qo nounset || oldOpts="+u"
+    set +u
     source @out@/bin/thisroot.sh
+    set "$oldOpts"
 }
 
 postHooks+=(thisroot)