[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.

Author: fs-eire

tools/ci_build/build.py

@@ -1228,9 +1228,28 @@ def generate_build_tree(
             ]
         env = {}
         if args.use_vcpkg:
-            env["VCPKG_KEEP_ENV_VARS"] = "TRT_UPLOAD_AUTH_TOKEN;EMSDK;EMSDK_NODE;EMSDK_PYTHON"
+            vcpkg_keep_env_vars = ["TRT_UPLOAD_AUTH_TOKEN"]
+
             if args.build_wasm:
                 env["EMSDK"] = emsdk_dir
+                vcpkg_keep_env_vars += ["EMSDK", "EMSDK_NODE", "EMSDK_PYTHON"]
+
+            #
+            # Workaround for vcpkg failed to find the correct path of Python
+            #
+            # Since vcpkg does not inherit the environment variables `PATH` from the parent process, CMake will fail to
+            # find the Python executable if the Python executable is not in the default location. This usually happens
+            # to the Python installed by Anaconda.
+            #
+            # To minimize the impact of this problem, we set the `Python3_ROOT_DIR` environment variable to the
+            # directory of current Python executable.
+            #
+            # see https://cmake.org/cmake/help/latest/module/FindPython3.html
+            #
+            env["Python3_ROOT_DIR"] = str(Path(os.path.dirname(sys.executable)).resolve())
+            vcpkg_keep_env_vars += ["Python3_ROOT_DIR"]
+
+            env["VCPKG_KEEP_ENV_VARS"] = ";".join(vcpkg_keep_env_vars)
 
         run_subprocess(
             [*temp_cmake_args, f"-DCMAKE_BUILD_TYPE={config}"],