MSBuild data should not be passed between tasks via static fields

[sharwell]

The following static field access should be rewritten using MSBuild inputs and outputs:

https://github.com/tonerdo/coverlet/blob/062b907735901bdf3ad9d4d40953f53a4fe3f364/src/coverlet.msbuild.tasks/CoverageResultTask.cs#L69

https://github.com/tonerdo/coverlet/blob/062b907735901bdf3ad9d4d40953f53a4fe3f364/src/coverlet.msbuild.tasks/InstrumentationTask.cs#L10

I believe the most straightforward and reliable implementation would involve generating GUIDs and/or temporary file names within MSBuild, and then passing those names to the build tasks. With this approach, the instrumentation task would produce known outputs for a given set of inputs, which can be tracked by the MSBuild targets.

📝 This limitation forces test execution to use /m:1 since the static property cannot keep track of information from multiple test executions in parallel.

[MarcoRossignoli]

Fixed in #409