chore: Fix post-submit coverage image.

Author: iphydf

.github/workflows/post-submit.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Build toxchat/c-toxcore:sources
         uses: docker/build-push-action@v4
         with:
-          file: other/docker/sources/Dockerfile
+          file: other/docker/sources/sources.Dockerfile
           tags: toxchat/c-toxcore:sources
       - name: Build and push
         uses: docker/build-push-action@v4