Switch from rustfmt-preview to rustfmt component in CI

We still have to stay at the nightly version of rustfmt as the `ignore`
option is not stable yet.
4 jobs for master in 10 minutes and 48 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Lint
passed #66000
rustfmt

00:00:48

 
  Test
passed #66001
test 1.28

00:10:00

passed #66003
allowed to fail
test nightly

00:09:08

passed #66002
test stable

00:09:45