try this

.gitea/workflows/pipeline.yml

@@ -23,7 +23,8 @@ jobs:
         run: |
           cd backend
           nix-shell -p elixir elixir-ls --run '
-            MIX_ENV=test mix do deps.get, test
+            MIX_ENV=test mix deps.get --only test
+            MIX_ENV=test mix test
           '
 
       - name: Build and push backend image