[3.12] gh-103053: Fix make check-clean-src: check "python" program (GH-110449) (#110453)

gh-103053: Fix make check-clean-src: check "python" program (GH-110449)

"make check-clean-src" now also checks if the "python" program is
found in the source directory: fail with an error if it does exist.
(cherry picked from commit a155f9f3427578ca5706d27e20bd0576f0395073)

Co-authored-by: Victor Stinner <vstinner@python.org>