automation: use latest AMIs
This cuts down on the overhead to create a new AMI, as we don't
have to install as many package upgrades.
Differential Revision: https://phab.mercurial-scm.org/D6916
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state