Remove artefacts from previous build at start of a branch build

Jenkins is currently configured to re-use workspaces between builds of
the same branch. Run `make clean` at the start of the build to ensure
that any artefacts from the previous build of the branch are removed.

Jenkinsfile

@@ -8,6 +8,7 @@ node {
 
     stage 'Build'
     nodeEnv.inside("-e HOME=${workspace}") {
+        sh 'make clean'
         sh 'make'
     }