Consider testing command lines to identify worker nodes

See https://github.com/Microsoft/msbuild/pull/3106#discussion_r176604238

Currently, in .NET Core, MSBuild will find _all_ processes running in the `dotnet` host and attempt to connect to them as though they were MSBuilds. That shouldn't interfere with the other processes unless they have a named pipe named `MSBuild{pid}`, but it takes time during the node-connection process.

We should explore narrowing down the candidate processes beforehand. We could for example examine process command lines for `/nodereuse:true` and/or `MSBuild.dll`.

Questions that need to be answered:

- [ ] How can this be done at all? AFAIK, there's no unified .NET API to get command lines of other processes on all platforms, so it might have to be platform-specific.
- [ ] Is this faster than the current approach? Examining command lines will take some time.