CI: don't retry jobs

We can't retry jobs blindly because if a build fails with warnings we want
to fail the job.  With retries enabled the job fails, but is immediately
retried and builds quickly from sstate without any warnings.

Thus, all and any warnings are hidden.

Disabling retries. We may get occasional failures from Docker, we'll have
to see how bad they are.

Change-Id: Ib726f14a264c029fdf372fc1b8a02aca52bf5e4c
Signed-off-by: Ross Burton <ross.burton@arm.com>