Auto merge of #3022 - jtgeibel:gh-actions-bionic, r=Turbo87
Auto merge of #3022 - jtgeibel:gh-actions-bionic, r=Turbo87

CI: Upgrade to ubuntu-18.04

This was missed in #1867 which only changed the frontend.

r? `@Turbo87`

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io