check-meta(hasUnsupportedPlatform): use lib.meta.availableOn

`hasUnsupportedPlatform` was not updated with #37395, so it does not
understand attrsets in `meta.[bad]platforms`.  In particular,
attrsets in `meta.badPlatforms` will "fail open" and be ignored.

Let's use `lib.meta.availableOn` instead of duplicating its logic.

Thanks to @alyssais for [noticing][1].

[1][https://github.com/NixOS/nixpkgs/pull/194148#discussion_r990817610]

Co-authored-by: sternenseemann <sternenseemann@systemli.org>

lib/meta.nix

@@ -98,9 +98,7 @@ rec {
        2. None of `meta.badPlatforms` pattern matches the given platform.
   */
   availableOn = platform: pkg:
-    if !(pkg?meta) then true else
-    (!(pkg.meta ? platforms) ||
-      lib.any (platformMatch platform) pkg.meta.platforms) &&
+    ((!pkg?meta.platforms) || lib.any (platformMatch platform) pkg.meta.platforms) &&
     lib.all (elem: !platformMatch platform elem) (pkg.meta.badPlatforms or []);
 
   /* Get the corresponding attribute in lib.licenses