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/pip-build-hook.sh

@@ -31,7 +31,7 @@ pipShellHook() {
     echo "Finished executing pipShellHook"
 }
 
-if [ -z "$dontUsePipBuild" ] && [ -z "$buildPhase" ]; then
+if [ -z "${dontUsePipBuild-}" ] && [ -z "$buildPhase" ]; then
     echo "Using pipBuildPhase"
     buildPhase=pipBuildPhase
 fi