ci: increase llvmpipe tests timeout from 4200 to 6200

Author: Italo Nicola

.github/workflows/build.yml

@@ -390,7 +390,7 @@ jobs:
           cd build
           export GGML_VK_VISIBLE_DEVICES=0
           # This is using llvmpipe and runs slower than other backends
-          ctest -L main --verbose --timeout 4200
+          ctest -L main --verbose --timeout 6200
 
   ubuntu-22-cmake-webgpu:
     runs-on: ubuntu-22.04