ci: server: fix python installation (ggml-org#6925)

Author: phymbert

.github/workflows/server.yml

@@ -106,7 +106,7 @@ jobs:
         run: |
           cd examples/server/tests
           python3 -m venv venv
-          source venv/bin/activate
+          . venv/bin/activate
           pip install -r requirements.txt
 
       - name: Tests