Provide environment variables overriding routing decision for tasks

### Summary

when reviewing https://github.com/dotnet/msbuild/pull/13660 it occured to me customers may want an escape hatch to use multithreaded mode with an unchangeable task that relies on static variables

### Background and Motivation

There are 3 places a task can run in multithreaded mode: in process taskhost, out of process taskhost that remains after build (sidecar), out of process taskhost that dies after build (transient)

- Ease of testing existing tests do complicated setups to get the correct taskhost
- routing tasks that you don't control but are confident they're MT-safe to get pref benefit

### Proposed Feature

provide environment variables that override routing decision by engine:
(there is precedent with env var `MSBUILDFORCEALLTASKSOUTOFPROC`)
```bash
MSBUILDFORCETASKSINPROCHOSTLIST="Fullname.T1,Fullnamespace.T2"
MSBUILDFORCETASKSTRANSIENTHOSTLIST="Fullname.T1,Fullnamespace.T2"
MSBUILDFORCETASKSSIDECARHOSTLIST="Fullname.T1,Fullnamespace.T2"
```

### Alternative Designs

_No response_