tests: use the right python when running dummyssh for narrow some plateform no longer have a `python` executable. Differential Revision: https://phab.mercurial-scm.org/D9504

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=$PYTHON "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF