fix(CMakeLists): install leanpkg
fix(CMakeLists): install leanpkg

fixes #1561

@jroesch Could you please check if this patch works on your OSX machine?

leanprover-community/lean,dselsam/lean,leodemoura/lean,Kha/lean,avigad/lean,leanprover-community/lean,fpvandoorn/lean,leanprover/lean,avigad/lean,dselsam/lean,digama0/lean,leanprover/lean,digama0/lean,rlewis1988/lean,leanprover/lean,fpvandoorn/lean,leanprover-community/lean,digama0/lean,avigad/lean,dselsam/lean,avigad/lean,digama0/lean,johoelzl/lean,dselsam/lean,leanprover/lean,leanprover-community/lean,johoelzl/lean,johoelzl/lean,leanprover/lean,Kha/lean,johoelzl/lean,leodemoura/lean,leanprover-community/lean,fpvandoorn/lean,Kha/lean,digama0/lean,leodemoura/lean,rlewis1988/lean,leodemoura/lean,fpvandoorn/lean,avigad/lean,Kha/lean,johoelzl/lean,johoelzl/lean,digama0/lean,dselsam/lean,dselsam/lean,leodemoura/lean,avigad/lean,leodemoura/lean,leanprover-community/lean,rlewis1988/lean,rlewis1988/lean,rlewis1988/lean,Kha/lean,rlewis1988/lean,leanprover/lean,Kha/lean,fpvandoorn/lean,fpvandoorn/lean