Make date invocation work with busybox

The `--iso-8601` flag is not supported in busybox date. It does support
the `-I` flag, which is also supported in the GNU coreutil `date`
implementation – they are in fact synonyms.

This improves portability ever so slightly.

rbuild

@@ -741,7 +741,7 @@ fi
 
 SECONDS=0
 RBUILD_SHOW_EXECUTION_TIME="true"
-RBUILD_STARTING_TIME="$(date --iso-8601=m | tr -d :)"
+RBUILD_STARTING_TIME="$(date -Iminutes | tr -d :)"
 
 main "$@"