[Doc] Update CMAKE_PREFIX_PATH for XPU windows README (pytorch#148863)

We found that the `pip install cmake` and `conda install cmake` has different behavior.
The reason is that the pip installed one doesn't find the corresponding libs under conda env. So we need to set the `CMAKE_PREFIX_PATH` for alignment.

Pull Request resolved: pytorch#148863
Approved by: https://github.com/CuiYifeng, https://github.com/malfet

Co-authored-by: Cui, Yifeng <[email protected]>

Author: 2 people

README.md

@@ -355,6 +355,16 @@ Please make sure [the common prerequisites](#prerequisites) as well as [the prer
 Then PyTorch can be built with the command:
 
 ```cmd
+:: CMD Commands:
+:: Set the CMAKE_PREFIX_PATH to help find corresponding packages
+:: %CONDA_PREFIX% only works after `conda activate custom_env`
+
+if defined CMAKE_PREFIX_PATH (
+    set "CMAKE_PREFIX_PATH=%CONDA_PREFIX%\Library;%CMAKE_PREFIX_PATH%"
+) else (
+    set "CMAKE_PREFIX_PATH=%CONDA_PREFIX%\Library"
+)
+
 python setup.py develop
 ```