Fix gRootDir for user-invoked rootcling.

TUnixSystem sets gRootDir to the returned char* of getenv().
A subsequent call to putenv("ROOTSYS=...") invalides the memory
pointed to by gRootDir.

When a user invokes rootcling, we can rely on TUnixSystem to extract
ROOTSYS; there is no need to do the same for rootcling_impl.