`hack/man-page-checker` fails on macOS: `\U` is a GNU sed extension

hack/man-page-checker:101 uppercases man page args with sed -E 's/_([a-z-]+)_/\U\1/g'. \U is GNU-only — BSD sed substitutes a literal U, so every SYNOPSIS vs --help comparison mismatches and make validate-docs fails on macOS with 11 bogus errors:

skopeo copy:   man='Usource-image Udestination-image' help='SOURCE-IMAGE DESTINATION-IMAGE'
skopeo delete: man='Uimage-name' help='IMAGE-NAME'

Reproduce on macOS:

make bin/skopeo BUILDTAGS=containers_image_openpgp
./hack/man-page-checker; echo "rc=$?"

CI misses it: doccheck runs in a Fedora container (GNU sed), and the osx job doesn't run validate-docs.