update apt repos before installing (#408)

This should hopefully resolve the recent errors in the github action
for running the fuzzers.

.github/workflows/fuzzers.yml

@@ -18,6 +18,7 @@ jobs:
     steps:
     - name: Install packages necessary for building
       run: |
+        sudo apt update
         sudo apt-get install --quiet ninja-build valgrind zip unzip
         wget https://apt.llvm.org/llvm.sh
         chmod +x llvm.sh