fix build-to-cachix

- Don't fail on `kill $cachixPid` when cachix has already exited.
  This fixes some failing CI builds.

- Stop the cachix background worker before the final `cachix push`.
  This can avoid unneeded reuploads.
  Use the coreutils version of tail on cirrus which supports
  argument --pid.

.cirrus.yml

@@ -39,4 +39,4 @@ task:
   build_script:
     - echo "sandbox = true" >> /etc/nix/nix.conf
     - export NIX_PATH="nixpkgs=$(nix eval --raw -f pkgs/nixpkgs-pinned.nix $nixpkgs)"
-    - nix run -f '<nixpkgs>' bash cachix -c ./ci/build.sh
+    - nix run -f '<nixpkgs>' bash coreutils cachix -c ./ci/build.sh