Do not delete temporary dir if DEBUG is set

To inspect the state of the application it is useful to checkout the
directory content of the temporary working directory. Setting the DEBUG
variable to either 1 or true stops bootstrap.sh to delete that directory,
which it does regardless if the app finished successfully or because of
an error.

bootstrap.sh

@@ -12,9 +12,13 @@ function cloneZSHLibRepo() {
   git submodule -q update --init --recursive --remote
 }
 
+function isDebug() {
+  test "${DEBUG}" -eq 1 -o "${DEBUG}" = true
+}
+
 function main() {
   local tmddir="`mktemp -d -t 'macos-system'`"
-  trap 'rm -fr -- "${tmpdir}"' EXIT
+  isDebug || trap 'rm -fr -- "${tmpdir}"' EXIT
   pushd -q "${tmddir}"
   cloneMacOSSystemRepo
   cloneZSHLibRepo