Install python3-lxml build dependencies in workflow

.github/workflows/im-build.yml

@@ -35,6 +35,7 @@ jobs:
     - name: Install dependencies
       run: |
         set -euxo pipefail
+        sudo apt-get build-dep python3-lxml
         python -m pip install --upgrade pip
         pip install -r requirements.txt
     - name: Install PostgreSQL client