Motivation for this change

I use remote builders to upgrade my laptop (nix.buildMachines option), and after a mass rebuild the final step that takes a lot of time looks like this (for the brevity of the example, I've interrupted the last step before running the switch again):

# nixos-rebuild switch
building Nix...
building the system configuration...
these derivations will be built:
  /nix/store/sacraxp35l3k0mcz92bmisf7rrqvxbdf-dbus-1.drv
  /nix/store/5ny75i0xcwic0plq30jrpjax9jfx5d5i-unit-dbus.service.drv
  /nix/store/2rwjbklbxzsgl6s6mnyrvj34svgdkyxk-system-units.drv
  /nix/store/s3p5db30hjp1aw847ml5q7xmsrrhr671-user-units.drv
  /nix/store/yxmxcclkchff3x18srpdz6h0aybb5qsz-etc.drv
  /nix/store/r8yw56k71dpn8wq30jp539kk5v7qaf4f-nixos-system-orivej-18.03.git.f575062.drv
copying 500 missing paths (5000 MiB) to ‘user@build-machine’...

All of this derivations prefer local build except dbus-1. If I enable it for dbus-1, the switch finishes instantaneously. Previously I had to rerun the switch with --no-build-hook when I wanted it to finish fast.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.