Merge pull request #56 from EFanZh/ci

Fetch updates before installing dependencies

.github/workflows/ci.yml

@@ -3,6 +3,7 @@ jobs:
     build:
         runs-on: ubuntu-latest
         steps:
+          - run: sudo apt-get update
           - run: sudo apt-get install --no-install-recommends -y latexmk lmodern texlive-latex-extra texlive-science
           - uses: actions/checkout@v2
           - run: make