Add trigger condition to block a plan from building if another plan is being executed

Customer scenario

Here is an example situation we see many times:

project build 1.0 runs sanity-test on agent a and connects to lab machine x
    project build 1.1 runs sanity-test on agent b and connects to lab machine x
    project build 2.0 runs sanity-test on agent c and connects to lab machine x

But...and this is the critical part, "lab machine x" can only be accessed by exactly one build plan at a time.