qa/crontab: check older builds on teuthology/nop when necessary

qa/crontab: check older builds on teuthology/nop when necessary

Today's scheduled run failed since the newest build of main
had failed. If we add `-n 10` to the command, this will
make it so we start at the newest build and backtrack
up to 10 older builds if necessary.

A higher number than that is not necessary, as the suite
failing to run will signal to us that more than the last
10 main builds are broken in Shaman.

Signed-off-by: Laura Flores <lflores@redhat.com>