[ci] Build without -m:1.

[jpobst]

Fix parallel build issues to allow us to build Java.Interop.sln without needing -m:1 on dotnet build.

Changes:

• Remove Java.Interop.BootstrapTasks.csproj from Java.Interop.sln and create a Java.Interop.BootstrapTasks.sln for -t:Prepare to run. This mirrors what XA does and keeps it from being rebuilt during the main run (at the same time other projects are trying to use Java.Interop.BootstrapTasks.dll).
• We need to add a Restore for Java.Interop.BootstrapTasks.csproj because it's no longer in Java.Interop.sln, but it has to be a separate invocation from the Build.
• Don't call GenerateVersionInfo target for Java.Interop.BootstrapTasks because it requires itself to already be built to run. It doesn't make much sense to version this assembly anyways since we do not ship it.
• Ordering our custom targets with BeforeTargets="BeforeBuild" is problematic, because it runs before referenced projects are built. Switch to Build or BeforeCompile as needed.

Tested in XA with: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=5651052&view=results.

[jonathanpeppers]

What you have here works -- and seems like the same approach we have in xamarin-android.

I also heard about this repo:

https://github.com/rainersigwald/build-task-in-solution-demo

Anywhere we are using "bootstrap" MSBuild tasks, we could potentially do:

https://github.com/rainersigwald/build-task-in-solution-demo/blob/17508937bb9edb0b817820146f85adf0da331515/Task/DemoFunctionality.targets#L4-L10

TaskFactory="TaskHostFactory" supposedly closes down the task after each call, and prevents file locking. Is it worth trying this out, and we could keep one .sln file?

[jpobst]

Oh, interesting. Maybe in the future I can use that to eliminate the "Prepare" step like I want to...

For now, since we have to have the Prepare step for other reasons I guess we can stick with this mechanism.

It sounds like the alternative would be a bit slower:

force MSBuild to load a new copy of the assembly in a new process for every execution of the task