images: add Arch Linux image

For testing with very new packages. It needs to be built nightly to avoid problems with outdated images, so the commit also adds a new is_nightly option.

It is used by knot-resolver!1546 (merged) for linters. The rationale here is that newer linters will usually catch more errors that affect old distros just as well as new ones, so I believe it is worth it to have them "less stable" than the rest of our pipelines.