treewide: Make still dont* Variables are optional in most cases

Go beyond the obvious setup hooks now, with a bit of sed, with a skipped case:

 - cc-wrapper's `dontlink`, because it already is handled.

Also, in nix files escaping was manually added.

EMP

pkgs/development/interpreters/python/hooks/pytest-check-hook.sh

@@ -43,7 +43,7 @@ function pytestCheckPhase() {
     echo "Finished executing pytestCheckPhase"
 }
 
-if [ -z "$dontUsePytestCheck" ] && [ -z "$installCheckPhase" ]; then
+if [ -z "${dontUsePytestCheck-}" ] && [ -z "$installCheckPhase" ]; then
     echo "Using pytestCheckPhase"
     preDistPhases+=" pytestCheckPhase"
 fi