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

ci/build-to-cachix.sh

@@ -13,11 +13,17 @@ trap 'echo Error at line $LINENO' ERR
 
 atExit() {
     rm -rf $tmpDir
-    if [[ -v cachixPid ]]; then kill $cachixPid; fi
+    if [[ -v cachixPid ]]; then stopCachix; fi
 }
 tmpDir=$(mktemp -d -p /tmp)
 trap atExit EXIT
 
+stopCachix() {
+    kill $cachixPid 2>/dev/null || true
+    # Wait for cachix to finish
+    tail --pid=$cachixPid -f /dev/null
+}
+
 ## Instantiate
 
 time nix-instantiate "$@" --add-root $tmpDir/drv --indirect > /dev/null
@@ -44,6 +50,7 @@ fi
 nix-build --out-link $tmpDir/result $tmpDir/drv >/dev/null
 
 if [[ $CACHIX_SIGNING_KEY ]]; then
+    stopCachix
     cachix push $cachixCache $outPath
 fi