ci: Prevent concurrent builds

The `disableConcurrentBuilds` pipeline option tells Jenkins to force
subsequent builds _of the same job_ to wait in queue until the one
running has completed.  This is sufficient when there is only one
branch/project in development at a time.  In order to prevent multiple
projects from running simultaneously, we need to acquire a global lock;
all projects need to have this same option in order for it to be
effective.

ci/Jenkinsfile

@@ -8,6 +8,11 @@ pipeline {
         }
     }
 
+    options {
+        disableConcurrentBuilds()
+        lock 'aimee-os'
+    }
+
     stages {
         stage('Prepare') {
             steps {