Set SDKROOT in macos job

Author: facundominguez

.github/workflows/build.yml

@@ -121,6 +121,10 @@ jobs:
         restore-keys: |
           repo-cache-${{ runner.os }}-nixpkgs-${{ env.cache-version }}-    
 
+    - name: Set SDK path
+      run: |
+      export SDKROOT=$(xcrun --sdk macosx --show-sdk-path)
+
     - name: Prefetch Stackage snapshot
       shell: bash
       run: |