[sca] python-%s-base addition breaks alpine-based apks

[kouhaidev]

https://github.com/chainguard-dev/melange/blob/3d4bb7ed71361e157eab839cb6d650067afdb032/pkg/sca/sca.go#L536-L537

for alpine-derived apks, this should either be skipped or append python3 instead

alpine doesn't ship python-%s-base bc thats a wolfi downstream change

this isnt the only notable ecosystem breakage: for at least 0.19.1-0.19.2 (latest) apko has shipped without support for local packages which breaks most third party users. https://github.com/chainguard-dev/apko/issues/1264

and a request: for the third party users out there, could you add tests using melange and apko together against alpine edge, just to make sure that things work? apko's been broken since aug 13.