CSmith GitHub action: run apt-get update

This is to fix failures to fetch a security update from Ubuntu mirrors.
Run `apt-get update` just like we do in other GitHub actions.

Also fix the Doxygen job configuration, which was also missing the
update step before package installation (but seemingly didn't fail thus
far).

Author: tautschnig

.github/workflows/csmith.yaml

@@ -18,6 +18,7 @@ jobs:
           # user input
           DEBIAN_FRONTEND: noninteractive
         run: |
+          sudo apt-get update
           sudo apt-get install --no-install-recommends -y build-essential flex bison maven ccache clang-10 clang++-10
           sudo apt-get install --no-install-recommends -y csmith libcsmith-dev
           make -C src minisat2-download

.github/workflows/doxygen-check.yaml

@@ -17,6 +17,7 @@ jobs:
         # user input
         DEBIAN_FRONTEND: noninteractive
       run: |
+        sudo apt-get update
         sudo apt-get install --no-install-recommends -yq doxygen graphviz
     - name: Run Doxygen
       run: ./scripts/run_doxygen.sh