log builder uid if building locally

[DieracDelta]

Motivation

It might be helpful to know the assigned builder for a drv. I talk about this more in #14639, but my specific usecase is tying processes to a builder for a nix task monitor. But I think this could also be helpful when debugging (for example, if a builder locks up and you want to figure out which derivation it was building)

Context

The context is also primarily #14639. This change feels relatively small to me, and shouldn't make much of a difference unless someone is parsing the log output.

In terms of testing I ran the check phase. Two tests fail, but they also seem to fail prior to this change (?)

I also ran the daemon and ran some local builds on both macos and nixos. Both targets printed the builder uid. But, I wasn't able to test a remote builder

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[edolstra]

This seems annoyingly verbose in the vast majority of times when you don't care about the UID of the builder. It's also not helpful when there are multiple concurrent builds, since the progress bar will show only one.

I'm more in favor of improving observability by allowing you to query the status of builds, e.g. having nix store info --json return information about all active builds (which could include their UIDs).