src/distbot.sh: Fix handling of multi-architecture repositories

src/distbot.sh: Fix handling of multi-architecture repositories

The conf/distributions file created by distbot for repositories with
multiple architectures specifies the repository's architectures with
a comma-separated list. However, the list should be space-separated.

This commit changes distbot so that
 * architectures must be passed one-by-one using `--arch`, and
 * the generated conf/distributions file contains a space-separated
   list of architectures.