ci: pin CUDA clone steps to ref: master

Without an explicit ref, actions/checkout makes a GitHub API call to
determine the default branch of leejet/stable-diffusion.cpp. With 14
parallel CUDA jobs all doing this simultaneously this call can fail,
presenting as a checkout error. Pinning to master skips the API call.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Phqen1x

.github/workflows/build.yml

@@ -117,6 +117,7 @@ jobs:
           submodules: recursive
           fetch-depth: 0
           repository: 'leejet/stable-diffusion.cpp'
+          ref: master
 
       - name: ccache
         uses: ggml-org/ccache-action@v1.2.16
@@ -217,6 +218,7 @@ jobs:
           submodules: recursive
           fetch-depth: 0
           repository: 'leejet/stable-diffusion.cpp'
+          ref: master
 
       - name: Install CUDA Toolkit
         id: cuda-toolkit