fix(shell/CMakeLists): remove lean executable before copying
fix(shell/CMakeLists): remove lean executable before copying

On windows, running executables cannot be overwritten.

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