ci: run nix jobs with --fallback

cache.nixos.org intermittently serves corrupt NARs; build from source on
substitute failure instead of failing the job.

.github/workflows/check-generated.yml

@@ -41,7 +41,7 @@ jobs:
 
       - name: Run make generate
         if: steps.changed-files.outputs.files == 'true'
-        run: nix develop --command -- make generate
+        run: nix develop --fallback --command -- make generate
 
       - name: Check for uncommitted changes
         if: steps.changed-files.outputs.files == 'true'