Prevent download of NARs we just uploaded

Dependencies

• [2] 6EO3HVNA Merge remote-tracking branch 'origin/master' into binary-cache
• [3] BAFICF73 Support hydra-build-products on binary cache stores
• [4] 73YR46NJ hydra-queue-runner: Write directly to a binary cache
• [5] FITVNQ2S Keep track of the time we spend copying to/from build machines
• [6] NAYQT2GT hydra-queue-runner: Use cmdBuildDerivation
• [7] MHVIT4JY Split hydra-queue-runner.cc more
• [8] A2GL5FOZ Moar stats
• [9] LE4VZIY5 More stats
• [10] 5AIYUMTB Basic remote building
• [11] DKJFD6JN Process Nix API changes
• [*] HJOEIMLR Refactor
• [*] N4IROACV Move buildRemote() into State

Change contents

• replacement in src/hydra-queue-runner/build-remote.cc at line 119
  [4.375]→[4.149:231](∅→∅),[4.438]→[4.149:231](∅→∅),[4.2504]→[4.149:231](∅→∅)
  − FdSource & from, FdSink & to, const PathSet & paths, counter & bytesReceived)
  [4.438]

  [4.2561]
  + FdSource & from, FdSink & to, const PathSet & paths, counter & bytesReceived,

  + std::shared_ptr<FSAccessor> accessor)
• replacement in src/hydra-queue-runner/build-remote.cc at line 124
  [4.2680]→[4.439:480](∅→∅)
  − destStore->importPaths(false, from);
  [4.2680]

  [4.232]
  + destStore->importPaths(false, from, accessor);
• edit in src/hydra-queue-runner/build-remote.cc at line 301
  [4.218]

  [4.218]
  + result.accessor = destStore->getFSAccessor();

  +
• replacement in src/hydra-queue-runner/build-remote.cc at line 305
  [4.273]→[2.64:134](∅→∅)
  − copyClosureFrom(destStore, from, to, outputs, bytesReceived);
  [4.273]

  [4.274]
  + copyClosureFrom(destStore, from, to, outputs, bytesReceived, result.accessor);
• replacement in src/hydra-queue-runner/builder.cc at line 129
  [4.4608]→[3.1967:2069](∅→∅)
  − if (result.success()) res = getBuildOutput(destStore, destStore->getFSAccessor(), step->drv);
  [4.4608]

  [4.4678]
  + if (result.success())

  + res = getBuildOutput(destStore, ref<FSAccessor>(result.accessor), step->drv);
• edit in src/hydra-queue-runner/state.hh at line 56
  [14.1810]

  [4.3661]
  + std::shared_ptr<nix::FSAccessor> accessor;