MSBuild caching

[adamralph]

With this change, the version is calculated once (per thread used by the SDK) and then cached.

For solutions with many projects, this can significantly reduce build time.

[adamralph]

@bording what do you think of this as a lightweight alternative to #989?

I was thinking it would be nice if MSBuild exposed tasks which could read and write from the build cache, so I went ahead and wrote those tasks. I could even see those potentially being contributed to MSBuild.

[adamralph]

FWIW, I tested this with https://github.com/Particular/ServiceControl, and a single threaded build went from 43 seconds to 24 seconds.

[adamralph]

testing the water with dotnet/msbuild#10418

[bording]

Overall I still prefer moving all the logic to an MSBuild task instead of calling other processes during the build, but this seems like an okay intermediate step.

[adamralph]

@bording I've released this in 5.1.0-rc.1—please feel free to give it a try.

[adamralph]

re-released as 6.0.0-rc.1 because the next release contains at least one breaking change