release-test: fetch host key for lille site

Tasks 4.5 and 4.6 use the firefly nodes at the lille site, so we need to
verify the host key for that side as well.

.github/workflows/release-test.yml

@@ -77,6 +77,12 @@ jobs:
         IOTLAB_USER=$(cat ~/.iotlabrc | cut -f1 -d:)
         ssh -oStrictHostKeyChecking=accept-new \
           ${IOTLAB_USER}@saclay.iot-lab.info exit
+    - name: Fetch host key from IoT-LAB lille site
+      if: ${{ matrix.pytest_mark == 'iotlab_creds' }}
+      run: |
+        IOTLAB_USER=$(cat ~/.iotlabrc | cut -f1 -d:)
+        ssh -oStrictHostKeyChecking=accept-new \
+          ${IOTLAB_USER}@lille.iot-lab.info exit
     - name: Checkout Release-Specs
       uses: actions/checkout@v2
       with: