test-as-submodule: ubuntu: apt-get update. (#777)

Author: olupton

.github/workflows/test-as-submodule.yml

@@ -42,6 +42,7 @@ jobs:
       - name: Install apt packages
         if: startsWith(matrix.os, 'ubuntu')
         run: |
+          sudo apt-get update
           sudo apt-get install bison cython3 flex libfl-dev libopenmpi-dev \
             ninja-build openmpi-bin python3-dev python3-numpy python3-pytest \
             python3-pytest-cov