packagePlatforms previously ignored badPlatforms, probably because it is
a fairly [recent] addition. While check-meta.nix doesn't implement it
using subtractLists, it is basically equivalent to the following logic:
platformSet - badPlatformSet (= effectivePlatformSet)
which we implement using subtractLists (which has somewhat confusing
currying-optimized argument order).
This flaw was discovered when testing #177901 which heavily uses
badPlatforms.
[recent]: b0482248fe
f4286483ee