Update the Linux CI environment to Ubuntu Bionic
Update the Linux CI environment to Ubuntu Bionic

asomers/nix,carllerche/nix-rust,carllerche/nix-rust,nix-rust/nix,nix-rust/nix,geofft/nix-rust,carllerche/nix-rust,geofft/nix-rust,geofft/nix-rust,asomers/nix