pkgs/top-level/release-outpaths.nix: omit attrnames which fail with "unsupported"

We have packages that use `meta.platforms = []` as a sort of synonym
for `broken = true`. Without this commit, the attrnames for those
jobs will end up in the list of attrnames which are expected to
build, even though they are not expected to build.

pkgs/top-level/release-outpaths.nix

···
             # hydra does not build unfree packages, so tons of them are broken yet not marked meta.broken.
             else if !includeBroken && builtins.elem reason [ "broken" "unfree" ]
             then throw "broken"
+            else if builtins.elem reason [ "unsupported" ]
+            then throw "unsupported"
             else true;
 
           inHydra = true;