Parallelize `maint/matrix-check`

maint/matrix-check runs multiple cargo check commands in serial, AFAICT. Running them in parallel may lead to faster CI times. We should first check what the CPU utilization is on the runner, though - if we're already using all the cores most of the time, making it more parallel won't help.

We also may be able to use the Gitlab CI parallel:matrix feature here, to launch different runners for each combination of values. I'm not sure if there's a reason that this wasn't done in the first place, though.