No clippy on macOS CI

[curiousleo]

Partial workaround for #258.

Overview

Cachix does not work on macOS (cachix/cachix#228) and Hydra does not successfully build clippy (NixOS/nixpkgs#77327, NixOS/nixpkgs#77358). As a result, in order to run clippy, we build it from scratch on every CI run on macOS. This adds about 20 minutes of build time.

There is no need to run clippy both on macOS and on Linux. So to circumvent this issue, this PR changes the CI setup slightly so that lints (including clippy) are only run on Linux.

Checklist

• Updated the documentation (code documentation, command help, ...)
• Tested the change (unit or integration tests)
• Amended the changelog in release.nix (see release.nix for instructions)

[curiousleo]

PTAL. The effect of this is to reduce the time nix-build -A allBuildInputs shell.nix > ./shell-inputs takes on macOS from 1244 seconds (20 minutes) to 95 seconds (1.5 minutes).

• before: https://travis-ci.org/target/lorri/jobs/634125087#L559 (roughly - this is from a different PR, but we don't seem to be running CI on master so this is as good a comparison as I can find right now)
• after: https://travis-ci.org/target/lorri/jobs/635266664#L558

[curiousleo]

@Profpatsch @grahamc PTAL

[grahamc]

Go ahead and rebase to resolve conflicts, regenerate the .yml and merge.