Build holds a 'master' executor while installers run

[AdamBrousseau]

Expected behaviour:

Shouldn't hold an executor while not doing work

https://github.com/adoptium/ci-jenkins-pipelines/blob/5247c3aacc24aeb5db6e225f54c99aaa1a03c1a4/pipelines/build/common/openjdk_build_pipeline.groovy#L601

Also not sure why only the mac case does a cleanup https://github.com/adoptium/ci-jenkins-pipelines/blob/5247c3aacc24aeb5db6e225f54c99aaa1a03c1a4/pipelines/build/common/openjdk_build_pipeline.groovy#L604

I guess there's other things that go on in the build*Installer functions that need master so this might not be as straight forward as I thought.

My issue is, I had the installer jobs backing up because some key machines went offline. Which caused a backup on master and blocked all builds once all the executors were full.