Require shellwords in artifact rake task (elastic#17319)

The elastic#17310 PR changed the rake task for
artifact creation to use shellwords from standard library. The acceptance tests
need to explitily load that library. This commit updates the rake file to handle
loading the required code.

Author: donoghuc

rakelib/artifacts.rake

@@ -15,6 +15,8 @@
 # specific language governing permissions and limitations
 # under the License.
 
+require 'shellwords'
+
 namespace "artifact" do
   SNAPSHOT_BUILD = ENV["RELEASE"] != "1"
   VERSION_QUALIFIER = ENV["VERSION_QUALIFIER"].to_s.strip.empty? ? nil : ENV["VERSION_QUALIFIER"]