ci: Use only two core

Author: jorisv

.github/workflows/macos-linux-windows-pixi.yml

@@ -109,6 +109,8 @@ jobs:
         submodules: recursive
 
     - uses: prefix-dev/[email protected]
+      env:
+        CMAKE_BUILD_PARALLEL_LEVEL: 2
       with:
         cache: true
         environments: test-pixi-build