ci: enable parallel building

Use `nix build --print-build-logs` to prefix the build output with the
name of its derivation.
This helps to distinguish build outputs when output is interleaved in
parallel builds.

.cirrus.yml

@@ -29,7 +29,7 @@ task:
           - scenario: netnsRegtest
       # This script is run as root
       build_script:
-        - echo "sandbox = true" >> /etc/nix/nix.conf
+        - printf '%s\n' 'sandbox = true' 'max-jobs = auto' >> /etc/nix/nix.conf
         - nix shell --inputs-from . nixpkgs#{bash,coreutils,cachix} -c ./test/ci/build.sh $scenario
 
     - name: flake

test/ci/build-to-cachix.sh

@@ -17,9 +17,10 @@ trap 'rm -rf $tmpDir' EXIT
 ## Instantiate
 
 time nix-instantiate "$@" --add-root "$tmpDir/drv" --indirect > /dev/null
-printf "instantiated "; realpath "$tmpDir/drv"
+drv=$(realpath "$tmpDir/drv")
+echo "instantiated $drv"
 
-outPath=$(nix-store --query "$tmpDir/drv")
+outPath=$(nix-store --query "$drv")
 if nix path-info --store "https://${cachixCache}.cachix.org" "$outPath" &>/dev/null; then
     echo "$outPath has already been built successfully."
     exit 0
@@ -33,12 +34,12 @@ fi
 
 if [[ $CACHIX_SIGNING_KEY ]]; then
     # Speed up task by uploading store paths as soon as they are created
-    buildCmd="cachix watch-exec $cachixCache nix-build --"
+    buildCmd="cachix watch-exec $cachixCache nix -- build"
 else
-    buildCmd=nix-build
+    buildCmd="nix build"
 fi
 
-$buildCmd --out-link "$tmpDir/result" "$tmpDir/drv" >/dev/null
+$buildCmd --out-link "$tmpDir/result" --print-build-logs "$drv^*"
 
 if [[ $CACHIX_SIGNING_KEY ]]; then
     cachix push "$cachixCache" "$outPath"