Fix make htmllive target (pythonGH-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 9a6e233)

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

Doc/Makefile

@@ -152,7 +152,7 @@ htmlview: html
 	$(PYTHON) -c "import os, webbrowser; webbrowser.open('file://' + os.path.realpath('build/html/index.html'))"
 
 .PHONY: htmllive
-htmllive: SPHINXBUILD = $(VENVDIR)/bin/sphinx-autobuild
+htmllive: SPHINXBUILD = PATH=$(VENVDIR)/bin:$$PATH sphinx-autobuild
 htmllive: SPHINXOPTS = --re-ignore="/venv/" --open-browser --delay 0
 htmllive: _ensure-sphinx-autobuild html