[3.13] Fix `make htmllive` target (GH-124219) (#124221)

Fix `make htmllive` target (GH-124219)

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)

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