[3.12] gh-123458: Skip SBOM generation if no git repository is detected (GH-123507) (#123615)

gh-123458: Skip SBOM generation if no git repository is detected (GH-123507)
(cherry picked from commit db42934270c5c23be9f6804cad98dfd8234caf6f)

Co-authored-by: Seth Michael Larson <seth@python.org>