scmutil: fix systemrcpath regression introduced in f5dd179bfa4a Changeset f5dd179bfa4a introduced a 'root' path component to look for hgrc files, which is used both as an absolute path and a path relative to the <install-root>. The latter one was broken since 'root' was set to an absolute location and the subsequent os.path.join discarded the <install-root> path prefix.