Allow setting PYTHON_LIB_REL_PATH via environment variable (#128419)

This allows builds to customize the location where caffe2's Python modules are installed to.

Fixes #ISSUE_NUMBER

Pull Request resolved: https://github.com/pytorch/pytorch/pull/128419
Approved by: https://github.com/PaliC, https://github.com/d4l3k, https://github.com/malfet

tools/setup_helpers/cmake.py

@@ -231,6 +231,7 @@ class CMake:
                     "SELECTED_OP_LIST",
                     "TORCH_CUDA_ARCH_LIST",
                     "TRACING_BASED",
+                    "PYTHON_LIB_REL_PATH",
                 )
             }
         )