Fix build against latest Nix

Dependencies

• [2] 3A7HTJZP Merge branch 'master' into libpqxx_undeprecate
• [3] 62MQPRXC Pass null values to libpqxx properly
• [4] WV4SSAIY Build against nix-master
• [5] UQQ4IL55 Add a error type for "unsupported system type"
• [6] KBZHIGLG Record the machine used for a build step
• [7] YTAYNN7V Queue monitor: Bail out earlier if a step has failed previously
• [8] DKR2GXSA Update libpqxx usage to move away from deprecated API interactions.
• [9] HTDANBLN Merge pull request #730 from NixOS/flake
• [10] 24BMQDZA Start of single-process hydra-queue-runner
• [11] IWB3F4Z6 Fail builds with previously failed steps early
• [12] NJXD2ABJ Abort unsupported build steps
• [13] MHVIT4JY Split hydra-queue-runner.cc more
• [14] DKJFD6JN Process Nix API changes
• [15] PQFOMNTL hydra-queue-runner: More stats
• [16] XIFDDTXZ Don't lock the BuildSteps table when inserting
• [17] U55WNIDP Abort unsupported build steps
• [18] EBJP3MNA Build against nix-master

Change contents

• replacement in src/hydra-queue-runner/builder.cc at line 469
  [3.3635]→[2.593:653](∅→∅)
  − for (auto & path : step->drv.outputPaths())
  [3.3635]

  [2.653]
  + for (auto & path : step->drv->outputPaths())
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 261
  [3.2860]→[3.2860:2889](∅→∅)
  − step->drv.platform,
  [3.2860]

  [3.2889]
  + step->drv->platform,
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 270
  [3.8997]→[2.820:864](∅→∅)
  − for (auto & output : step->drv.outputs)
  [3.8997]

  [3.3251]
  + for (auto & output : step->drv->outputs)
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 455
  [3.1390]→[2.1105:1153](∅→∅)
  − for (auto & path : step->drv.outputPaths())
  [3.1390]

  [2.1153]
  + for (auto & path : step->drv->outputPaths())
• replacement in src/hydra-queue-runner/queue-monitor.cc at line 175
  [3.7928]→[2.1271:1340](∅→∅)
  − localStore->printStorePathh(ex.step->drvPath));
  [3.7928]

  [3.7968]
  + localStore->printStorePath(ex.step->drvPath));
• replacement in src/hydra-queue-runner/queue-monitor.cc at line 179
  [3.1276]→[2.1343:1408](∅→∅)
  − for (auto & output : ex.step->drv.outputs) {
  [3.1276]

  [3.8047]
  + for (auto & output : ex.step->drv->outputs) {