jobs/build: don't start new build until previous build completes

Since 973bcf9 ("jobs/build: rerun `build-arch` if previous build is
incomplete"), there is a race possible where the `build` job rerun logic
could kick in before the `release` jobs initially triggered for that
build has finished. We don't want to queue builds in that case.

Let's just prevent starting up the build job at all if there are
remaining pieces from another build for this stream running.

jobs/build.Jenkinsfile

@@ -121,6 +121,15 @@ if (params.WAIT_FOR_RELEASE_JOB) {
     timeout_mins += timeout_mins + 30
 }
 
+// Before proceeding let's make sure no jobs for this stream are currently
+// running. This includes multi-arch jobs and release jobs.
+echo "Waiting for a clear runway for ${params.STREAM}"
+def locks = additional_arches.collect{[resource: "build-${params.STREAM}-${it}"]}
+locks += ["release-${params.STREAM}"]
+lock(resource: "build-${params.STREAM}", extra: locks) {
+    echo "The runway is clear for ${params.STREAM}"
+}
+
 lock(resource: "build-${params.STREAM}") {
     timeout(time: timeout_mins, unit: 'MINUTES') {
     cosaPod(cpu: "${ncpus}",