Create script to easily remove existing CI build artifacts.

etc/ci/clean_build_artifacts.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+rm -rf target/{debug,release,doc,geckolib}