Add a block to prevent multiple workload install, uninstall, repair, restore, or upgrade steps from happening at the same time

[marcpopMSFT]

No description provided.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[wli3]

@dsplaisted

We don't advertise .NET SDK to be able to run in parallel.

Doing lock this late makes me nervous. If there is a bug -- for example the lock file on disk does not get cleaned up, so .NET SDK is locked forever -- it is usually hard to discover

[dsplaisted]

When we discussed it in one of our workloads meetings, we were worried that if two installations did end up running in parallel, we might get into an unrecoverable state. So we wanted to protect against this.

I think we said that we can use a mutex instead of a lock file, which I think will automatically be released when the process exits.