Remove /etc as hardcoded sysconfdir from Nix build

[dominiklohmann]

📔 Description

This PR contains two changes:

• Our static binaries behaved differently than our regular relocatable installations: They were looking for the global configuration in /etc/vast/vast.yaml rather than the expected <prefix>/etc/vast/vast.yaml. This change makes them behave just like regular relocatable installations.
  For users, this means that configuration files at /etc/vast/vast.yaml need to be moved accordingly.
• We now install an example config file alongside VAST.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Read the code, try the static binary from CI.

[mavam]

Why is the example config all commented out?

[dominiklohmann]

Why is the example config all commented out?

That's intentional: It's an example config file, and if we install it, then we don't want it to mesh with the actual defaults. Unless we find a way to guarantee that defaults and example config are in sync (which we don't right now), I'd rather have everything commented out.

[mavam]

I would agree if it's an actual .yaml file. But it's an .example file that VAST doesn't look at anyway on startup.

The UX of reading a properly syntax-highlighted file and separate the tuning knobs from the content is not to be dismissed, I think.

[dominiklohmann]

I would agree if it's an actual .yaml file. But it's an .example file that VAST doesn't look at anyway on startup.

Take a look at the changed CMake code in this PR: We install this file to <prefix>/etc/vast/vast.yaml.

[mavam]

I wonder whether we should simply keep the uncommented example file in the repo and install a transformation (e.g., through sed) at the destination. That would make at least maintaining the example config file easier and avoid mistakes.

[mavam]

If you can get awk installed in Nix, we're in business in CI.