CI: Fix package installation for Coverity run

Something has changed recently in the Github Actions environment and the
golang metapacakge resolves to something that no longer exists:

  https://github.com/xen-project/xen/actions/runs/11539340171/job/32120834909

Update the apt package index before installing, which fixes things.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

.github/workflows/coverity.yml

@@ -12,6 +12,7 @@ jobs:
     steps:
     - name: Install build dependencies
       run: |
+        sudo apt-get update
         sudo apt-get install -y \
           build-essential \
           git-core \