[build] ensure Python to be found in vcpkg (#24713)
### Description
<!-- Describe your changes. -->
Sometimes Python cannot be found by CMake when using vcpkg. This is
because vcpkg does not inherit environment `PATH` from the parent
process when launching CMake (which is intentionally done by vcpkg), and
in this case, a non-standard Python installation (eg. virtual env or
conda) may fail the `find_package(Python3 REQUIRED)`.
This PR write the env variable `Python3_ROOT_DIR` and adds it to
`VCPKG_KEEP_ENV_VARS` to ensure CMake launched by vcpkg to be able to
always find Python that runs `build.py`.
This change should not break the existing behavior unless there are
scenarios that intentionally uses different python for running build.py
and vcpkg build.
