Unixify path in t/INSTALL_BASE.t.

98fdebd96dd broke the test on VMS because rel2abs converts the
directory to a native path, which we then paste Unix-style path
components onto, resulting in an invalid mogrel path.

N.B.  We can't just call File::Spec::Unix->rel2abs because that
calls $self->canonpath, which, on VMS, also converts the path to
native format.