Unreserve doesn't set environment variable

The manual reserve feature doesn't work well with the pipeline variable feature. When a user unreserves a resource and the plugin assigns the resource immediately to a queued build, the environment variable won't be correctly set.

In LockableResourceManager.unreserve, there is a LockStepExecution.proceed call at the end with the variable left null. I have no idea how to retrieve a singular variable to use at this point in the code.