packaging: builddeb's cleanup needs to expand PWD, safely

Single quotes would not expand the variable.

contrib/packaging/builddeb

@@ -16,7 +16,7 b' DEBFLAGS=-b'
 
 cleanup() {
     if [ '$CLEANUP' ]; then
-        rm -r '$PWD/debian';
+        rm -r "$PWD/debian";
     fi
 }