GH Actions: run apt-get update before installing packages

See #64 (comment)

.github/workflows/build.yml

@@ -20,6 +20,7 @@ jobs:
       - uses: actions/checkout@v4
       - name: restore
         run: |
+          sudo apt-get update
           sudo apt-get install -y libgtest-dev
       - name: build
         env: