Fix setting Python3_ROOT_DIR on Windows

Summary:
Previously the value of Python3_ROOT_DIR was set to the string
"PYTHON_HOME" instead of the value of the variable named
PYTHON_HOME. This commit fixes that as CMake expects
a path as the value of Python3_ROOT_DIR

Reviewers: #lldb, JDevlieghere, teemperor

Reviewed By: #lldb, JDevlieghere, teemperor

Subscribers: teemperor, JDevlieghere, mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77842