Fix root creation when the root already exists but is owned by another user

Dependencies

• [2] YXYXJDMB hydra-queue-runner: Write GC roots for outputs paths
• [3] BRAESISH Warn if PostgreSQL appears stalled
• [4] MHVIT4JY Split hydra-queue-runner.cc more
• [*] 24BMQDZA Start of single-process hydra-queue-runner
• [*] EPWEMRI2 Allow determinism checking for entire jobsets
• [*] HJOEIMLR Refactor

Change contents

• replacement in src/hydra-queue-runner/builder.cc at line 267
  [2.52]→[2.52:114](∅→∅)
  − writeFile(rootsDir + "/" + baseNameOf(path), "");
  [2.52]

  [2.114]
  + addRoot(path);
• edit in src/hydra-queue-runner/builder.cc at line 476
  [3.14591]
  +

  +

  + void State::addRoot(const Path & storePath)

  + {

  + auto root = rootsDir + "/" + baseNameOf(storePath);

  + if (!pathExists(root)) writeFile(root, "");

  + }
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 843
  [7.2402]

  [7.2402]
  +

  + addRoot("/nix/store/rk9lahzm21qm0zjclv6mz0m4hy77wk14-vm-test-run-unnamed.drv");
• replacement in src/hydra-queue-runner/queue-monitor.cc at line 225
  [2.301]→[2.301:367](∅→∅)
  − writeFile(rootsDir + "/" + baseNameOf(path), "");
  [2.301]

  [2.367]
  + addRoot(path);
• edit in src/hydra-queue-runner/state.hh at line 537
  [8.9109]

  [8.9109]
  +

  + void addRoot(const nix::Path & storePath);