ci: fix flake check in newer Nix version

I don't know what's changed, but apparently adding a fragment specifier
fixes the CI errors..

Author: nbdd0121

.github/workflows/ci.yml

@@ -43,4 +43,4 @@ jobs:
           nix build -L .?submodules=1#sonata-tests
 
       - name: Run Nix Checks
-        run: nix flake check -L .?submodules=1
+        run: nix flake check -L .?submodules=1#