hydra-queue-runner: Improve SSH flags

Dependencies

• [2] WKJFPR77 hydra-queue-runner: Maintain count of active build steps
• [3] ZSBPKI2P Pass ssh key
• [4] 5AIYUMTB Basic remote building

Change contents

• edit in src/hydra-queue-runner/build-remote.cc at line 42
  [3.917]→[2.0:45](∅→∅)
  − // FIXME: ensure no password prompt.
• replacement in src/hydra-queue-runner/build-remote.cc at line 43
  [2.83]→[3.0:107](∅→∅),[3.917]→[3.0:107](∅→∅)
  − Strings argv({"ssh", sshName, "-i", sshKey, "-x", "-a", "--", "nix-store", "--serve", "--write"});
  [2.83]

  [3.1010]
  + Strings argv(

  + { "ssh", sshName, "-i", sshKey, "-x", "-a"

  + , "-oBatchMode=yes", "-oConnectTimeout=60", "-oTCPKeepAlive=yes"

  + , "--", "nix-store", "--serve", "--write" });