chromium: try to hack around Hydra problems

Discussion: https://github.com/NixOS/nixpkgs/commit/e8f1ddcbd1d

pkgs/applications/networking/browsers/chromium/common.nix

@@ -228,9 +228,14 @@ let
     '';
 
     buildPhase = let
+      # Build paralelism: on Hydra the build was frequently running into memory
+      # exhaustion, and even other users might be running into similar issues.
+      # -j is halved to avoid memory problems, and -l is slightly increased
+      # so that the build gets slight preference before others
+      # (it will often be on "critical path" and at risk of timing out)
       buildCommand = target: ''
         ninja -C "${buildPath}"  \
-          -j$NIX_BUILD_CORES -l$NIX_BUILD_CORES \
+          -j$(( ($NIX_BUILD_CORES+1) / 2 )) -l$(( $NIX_BUILD_CORES+1 )) \
           "${target}"
       '' + optionalString (target == "mksnapshot" || target == "chrome") ''
         paxmark m "${buildPath}/${target}"