Auto merge of #60665 - pietroalbini:upgrade-ci-images, r=alexcrichton
Upgrade non-LTS Ubuntu images in CI
This PR bumps `dist-various-2` to 18.04 LTS and both `x86_64-gnu` and `x86_64-gnu-debug` to 19.04.
Another change is the switch to [rust-lang/mirror-google-fuchsia-zircon](https://github.com/rust-lang/mirror-google-fuchsia-zircon) as the Zircon repository, used during Fuchsia tests: the original repository ([fuchsia.googlesource.com/zircon](https://fuchsia.googlesource.com/zircon)) is now behind a login wall and it doesn't contain anything at all.
r? @alexcrichton
cc @cramertj -- what's happening on the Fuchsia side?