Update actions/cache to v4

Author: mmp

.github/workflows/ci-gpu.yml

@@ -53,7 +53,7 @@ jobs:
         path: build
 
     - name: Cache OptiX headers
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: optix
         key: optix-7.12345