s3backup: Create temporary file in temp dir

Dependencies

• [2] UGA45FNC Add a plugin for backing up builds in s3
• [3] UGFTLRWC Delay initializing the s3 client until it's needed to silence warnings

Change contents

• replacement in src/lib/Hydra/Plugin/S3Backup.pm at line 71
  [2.3818]→[2.3818:3894](∅→∅)
  − my $tempdir = File::Temp->newdir("s3-backup-nars-$build_id" . "XXXXX");
  [2.3818]

  [2.3894]
  + my $tempdir = File::Temp->newdir("s3-backup-nars-$build_id" . "XXXXX", TMPDIR => 1);