Evaluate making the _RunRidSpecificBuild target run the rid-specific builds in parallel

### Evaluate making the `_RunRidSpecificBuild` target run the rid-specific builds in parallel

The inner MSBuild task in the `_RunRidSpecificBuild` for reference: https://github.com/dotnet/macios/blob/bf8f47221475201915c3a41ae5db7e09222c0c79/dotnet/targets/Xamarin.Shared.Sdk.targets#L393-L407

We could make this task run the inner `MSBuild` tasks in parallel.

Potential pros:

* Faster build due to parallelization.
* Find bugs due to logic errors / double writes, etc.

Potential cons:

* If it's not safe to run rid-specific builds in parallel, any bugs are likely to be random and hard to diagnose.
* Slower build due to CPU/memory thrashing (we already AOT-compilation for instance to max out the CPU, doing it twice will make the build slower).

Things to consider:

* Only release builds are multi-rid, and we don't care as much about speed for release builds as we do for debug builds.