Don't rely on ${LOGNAME:-foo} evaluating to a string

Dependencies

• [2] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [3] IZYR3KTL release.nix: Get rid of the "system" argument
• [4] 3CSPTBW5 Fix perl search path; avoid reference to build path (#471)
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 160
  [2.3516]→[2.3516:3555](∅→∅)
  − export LOGNAME=${LOGNAME:-foo}
  [2.3516]

  [2.3555]
  + export LOGNAME=''${LOGNAME:-foo}