Bump memory limit a bit

Dependencies

• [2] SL3WSRAC hydra-queue-runner: Limit memory usage
• [3] 24BMQDZA Start of single-process hydra-queue-runner
• [4] CNLNT3T4 Allow only 1 thread to send a closure to a given machine at the same time
• [5] GJV2J5HX Pool local store connections
• [6] ENXUSMSV Make concurrency more robust
• [7] B2L4T3X6 Sync with Nix
• [8] HJOEIMLR Refactor

Change contents

• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 20
  [3.7501]→[2.1355:1419](∅→∅)
  − : memoryTokens(4ULL << 30) // FIXME: make this configurable
  [3.7501]

  [3.7501]
  + : memoryTokens(6ULL << 30) // FIXME: make this configurable