ci: Clean workspace after every stage

Clean the workspace after every stage (ie, test) to avoid filling
disk.  The downside of this change is that we can't reuse a checkout
of OMPI between stages that run on the same build node.  The upside
is that we are much less likely to run out of disk space during a
test.  We ran into some issues today when there were many builds,
because the workspace name is different between pull requests, and
when a build node had enough checkouts (one for each pull request),
we filled the disk.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

Author: bwbarrett

.ci/community-jenkins/Jenkinsfile

@@ -80,6 +80,7 @@ def prepare_build(build_name, label, build_arg) {
             node(label) {
                 checkout(changelog: false, poll: false, scm: scm)
                 sh "/bin/bash -x .ci/community-jenkins/pr-builder.sh ${build_arg} ompi"
+                cleanWs(notFailBuild: true)
             }
         }
     }