system: Default .bashrc uses '--color=auto' for grep and ls.

Reported by Tobias Geerinckx-Rice <me@tobias.gr>
and Björn Höfling <bjoern.hoefling@bjoernhoefling.de>.

* gnu/system/shadow.scm (default-skeletons)["bashrc"]: Use
'--color=auto' instead of '--color'.