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-install-hook.sh

@@ -18,7 +18,7 @@ pipInstallPhase() {
     echo "Finished executing pipInstallPhase"
 }
 
-if [ -z "$dontUsePipInstall" ] && [ -z "$installPhase" ]; then
+if [ -z "${dontUsePipInstall-}" ] && [ -z "$installPhase" ]; then
     echo "Using pipInstallPhase"
     installPhase=pipInstallPhase
 fi