GHA: Do apt-get update before adding repos

There are failures like
> 	Adding repository ppa:ubuntu-toolchain-r/test
> Error: retrieving gpg key timed out.

So add an intermediate update to avoid this.

.github/workflows/ci.yml

@@ -206,6 +206,8 @@ jobs:
             SOURCES+=(ppa:ubuntu-toolchain-r/test)
 
             ci/add-apt-keys.sh "${SOURCE_KEYS[@]}"
+            # Initial update before adding sources required to get e.g. keys
+            sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT update
             ci/add-apt-repositories.sh "${SOURCES[@]}"
 
             sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT update