v2021 ← v2022dev2 backport: Run apt update before trying to download the package source

Since infrastructure/apertis-image-recipes@7252fff4 the devroot no longer has valid entries under /var/lib/apt/lists, so apt source no longer works out-of-the-box.

Run apt update to refresh the listings before downloading the sources for the package to be built.

See !2 (merged)