Doc: do update before fetching packages in WSL build guide #12668

Add update and upgrade commands to enable the installation of the first dependencies on ubuntu xenial. If those are not executed some packages can not be found.

Indeed the apt-get can fail on a system with an old package catalog, due to missing packages. Note that you don't strictly need the apt-get upgrade, just the update. But it's ok...

The concern I have with running the upgrade/update is that it might have an effect on other packages not related to Bitcoin. Might it be better to instead add a note that "apt-get update" might help in the event that some dependencies are missing?

I ran into this problem when using the WSL and after doing a fresh install of Ubuntu from the Microsoft store. So I would think that everybody using this workflow will run into this problem.

I also ran into some other problems for which I haven't created a pull request yet: mainly missing packages and the installation of berkleydb 4.8. So maybe a seperate section for a freshly installed Ubuntu Xenial on WSL would be appropriate.

The concern I have with running the upgrade/update is that it might have an effect on other packages not related to Bitcoin.

I understand the concern but also note that this is always the case. When you apt-get install a package you get its most recent version, as well as the most recent of its dependencies, and the dependencies in turn, all the way up to base system libraries.

So maybe a seperate section for a freshly installed Ubuntu Xenial on WSL would be appropriate.

FWIW there's nothing specific to Xenial in this PR. Running apt-get update before installing packages makes as much sense for every Debian variant in existence.

utACK e29c6c8 The Windows build instructions seem fragile at the best of times, if running apt update/upgrade is going to cause more issues that's worrying.

I've changed the PR title to be clearer.