[Nix-dev] Hydra Building PRs

[Graham Christensen]

Update:

Before now, PRs were being built using the nixpkgs release.nix.

As of
https://github.com/grahamc/hydra-prs/commit/df790c5c7f36bc0b0f033b1caf9c3188ddfa4065
all builds are being performed now on the NixOS release-combined.nix.

This means each push will trigger affected tests to run in their VMs.
Any outstanding PRs will have their configuration updated before their
next evaluation. ie: If you have a PR open already, and you make a
change to it, it'll start using this new configuration.

SIDE NOTE: I'm considering making a bot which comments on PRs with a
link to their jobset. Unless someone objects, I'll go ahead and do so.

Graham