Use local variable value for trap at parse time

`trap` can not access the value of a variable defined as local once the
function body is left. So we use the value of the variable at parse time
instead of trying to access the value at runtime of the `trap` command.

bootstrap.sh

@@ -18,7 +18,7 @@ function isDebug() {
 
 function main() {
   local tmddir="`mktemp -d -t 'macos-system'`"
-  isDebug || trap 'rm -fr -- "${tmpdir}"' EXIT
+  isDebug || trap "rm -fr -- '${tmpdir}'" EXIT
   pushd -q "${tmddir}"
   cloneMacOSSystemRepo
   cloneZSHLibRepo