[3.12] Fix `make htmllive` target (GH-124222)

Allow `make -C Doc htmllive` to work without manual venv activation

Set PATH to ensure that `sphinx-autobuild` can find `sphinx-build`.

(cherry picked from commit 9a6e2336e4b54fc13064b77826a67b03b3b45133)

Authored-by: Zachary Ware <zach@python.org>