ci: execute "apt update" before the first "apt install"

Apparently, the base image of the Github CI runner is not update very often, and
does not keep up with the evolving list of mirrors.

Without this change the "apt install" command starts to hit 404s.

.github/workflows/test-itcoin-core.yml

@@ -14,7 +14,7 @@ jobs:
     steps:
       - name: Install the build toolchain (gcc-12)
         run: |
-          sudo apt install --no-install-recommends -y \
+          sudo apt update && apt install --no-install-recommends -y \
               autoconf \
               automake \
               bsdextrautils \