Add an apt-update step to the docker pull request check.

This stops it from getting an HTTP 404 while downloading
dependencies if they have been updated since the image
came out.

Author: NlightNFotis

.github/workflows/pull-request-checks.yaml

@@ -365,7 +365,9 @@ jobs:
         with:
           submodules: recursive
       - name: Download test dependencies
-        run: sudo apt install openjdk-11-jdk-headless
+        run: |
+          sudo apt update
+          sudo apt install openjdk-11-jdk-headless
       - name: Build docker image
         run: docker build -t cbmc .
       - name: Smoke test goto-cc