Change Clever Cloud flavours
The XS flavour is just too slow, often failing to run the jobs within
reasonable time. Instead, run most jobs under the M flavour, which has
two CPUs and more memory. For fast feedback during development, run the
“Current” jobs under the L flavour with its four cores.
As a potential offset for the extra requirements, don't retry jobs automatically.
Minor, infrastructure-related jobs that just push stuff remain under XS.
(One of the tests still seemed to fail, so I've marked it as slow.)