Tweak Windows build parallelization settings (#19664)
### Description
<!-- Describe your changes. -->
Use UseMultiToolTask and limit the number of cl.exe instances running.
MultiToolTask info:
https://devblogs.microsoft.com/cppblog/improved-parallelism-in-msbuild/
Info on why limiting CL_MPCount can help:
https://github.com/Microsoft/checkedc-clang/wiki/Parallel-builds-of-clang-on-Windows
The current CIs have 4 cores (both physical and logical). Hardcoded the
GPU build in win-ci.yml to use CL_MPCount of 2 as that seems to work
fine. Can adjust if needed to base it on the actual number of cores or
to use build.py to build.
Caveat: I've run about 16 builds and haven't seen a slow build yet, but
as the root cause of the slow builds isn't really known this isn't
guaranteed to be a fix.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Try and prevent super slow GPU builds by reducing number of tasks
potentially running in parallel.
