Use configured compression in hydra-compress-logs service, drop mtime

When the service would fail for longer than 3 hours (very likely), then
old logs would never be compressed

nixos-modules/hydra.nix

@@ -462,7 +462,9 @@ in
       { path = [ pkgs.bzip2 ];
         script =
           ''
-            find ${baseDir}/build-logs -type f -name "*.drv" -mtime +3 -size +0c | xargs -r bzip2 -v -f
+            set -eou pipefail
+            compression=$(sed -nr 's/compress_build_logs_compression = ()/\1/p' ${baseDir}/hydra.conf)
+            find ${baseDir}/build-logs -type f -name "*.drv" -size +0c | xargs -r $compression -v -f
           '';
         startAt = "Sun 01:45";
       };