contrib/ci: do not upgrade Arch continuously
contrib/ci: do not upgrade Arch continuously

`yu` was added in 014e5526ff4f8430c7709eb2b8094f0767af8437 to solve cache issues.

But since then several other invocations of pacman have been added and doing so constantly is pointless, as you are unlikely to see new upgrades while the CI is running (and it might not be desired either). It also breaks testing older versions of fwupd as seen in GH-4860. So upgrade only once at the beginning and keep installing from the same cache afterwards.
fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd