S3Backup: Create a proper nar

Dependencies

• [2] UGA45FNC Add a plugin for backing up builds in s3
• [3] R6PCOVLX s3backup: Create temporary file in temp dir

Change contents

• replacement in src/lib/Hydra/Plugin/S3Backup.pm at line 99
  [2.5144]→[2.5144:5252](∅→∅)
  − system("$Nix::Config::binDir/nix-store --export $path $compressor > $tempdir/nar") == 0 or die;
  [2.5144]

  [2.5252]
  + system("$Nix::Config::binDir/nix-store --dump $path $compressor > $tempdir/nar") == 0 or die;