melange: Incomplete package integrity verification allows data section substitution

melange allows users to build apk packages using declarative pipelines. Apko prior to version 1.2.9, corresponding to melange prior to version 0.50.4, verified the control section hash (`.PKGINFO` etc.) against the signed `APKINDEX`, but never verified the data section hash (the actual package files that get installed). An attacker who could compromise a mirror, poison a cache, or MITM a package fetch could substitute arbitrary file contents while the control hash check still passed. Apko version 1.2.9 and melange version 0.50.4 contain a fix.