feat(ci): respect dont_clean

When dont_clean is set we want to leave the workspace as it is. When
that happens we also need for things to be roughly idempotent so they
can run again and again.

This allows for builds in about 15% of the clean build time.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I872216aac2442b637194f4c19138d60d061078bd
diff --git a/script/build_package.sh b/script/build_package.sh
index c4a7a7f..f9ac52e 100755
--- a/script/build_package.sh
+++ b/script/build_package.sh
@@ -1366,7 +1366,7 @@
 	echo "===== Building package in mode: $mode ====="
 	# Build with a temporary archive
 	build_archive="$archive/$mode"
-	mkdir "$build_archive"
+	mkdir -p "$build_archive"
 
 	if [ "$mode" = "debug" ]; then
 		export bin_mode="debug"