Check non-runnable steps for unsupported system type

Dependencies

• [2] N5O7VEEO Immediately abort builds that require an unsupported system type
• [3] 5AIYUMTB Basic remote building
• [4] ENXUSMSV Make concurrency more robust
• [5] 2IQRXLWE Support cancelling builds
• [6] FQQRJUO4 Mark builds as busy
• [7] 24BMQDZA Start of single-process hydra-queue-runner
• [8] UQQ4IL55 Add a error type for "unsupported system type"
• [9] NJJ7H64S Very basic multi-threaded queue runner

Change contents

• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 227
  [3.1249]→[3.1249:1293](∅→∅)
  − std::set<Step::ptr> & newRunnable);
  [3.1249]

  [3.7091]
  + std::set<Step::ptr> & newSteps, std::set<Step::ptr> & newRunnable);
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 476
  [3.11027]→[3.2953:3067](∅→∅)
  − std::set<Step::ptr> newRunnable;

  − Step::ptr step = createStep(store, build->drvPath, newRunnable);
  [3.11027]

  [3.3067]
  + std::set<Step::ptr> newSteps, newRunnable;

  + Step::ptr step = createStep(store, build->drvPath, newSteps, newRunnable);
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 498
  [2.430]→[2.430:469](∅→∅)
  − for (auto & r : newRunnable) {
  [2.430]

  [2.469]
  + for (auto & r : newSteps) {
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 534
  [3.3558]→[3.9950:10110](∅→∅)
  − printMsg(lvlInfo, format("added build %1% (top-level step %2%, %3% new runnable steps)")

  − % build->id % step->drvPath % newRunnable.size());
  [3.3558]

  [3.10110]
  + printMsg(lvlInfo, format("added build %1% (top-level step %2%, %3% new steps, %4% new runnable steps)")

  + % build->id % step->drvPath % newSteps.size() % newRunnable.size());
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 575
  [3.4065]→[3.4065:4104](∅→∅)
  − std::set<Step::ptr> & newRunnable)
  [3.4065]

  [3.11564]
  + std::set<Step::ptr> & newSteps, std::set<Step::ptr> & newRunnable)
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 595
  [3.11832]

  [3.11832]
  + newSteps.insert(step);
• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 615
  [3.12329]→[3.4630:4695](∅→∅)
  − Step::ptr dep = createStep(store, i.first, newRunnable);
  [3.12329]

  [3.12374]
  + Step::ptr dep = createStep(store, i.first, newSteps, newRunnable);