I poked around a bit more, and it looks like a fair number of the installation messages are coming from `Lib/distutils/command/build_scripts.py`, which is using `distutils.log` to show what it's doing:

      log.debug("not copying %s (up-to-date)", script)
...
      log.info("copying and adjusting %s -> %s", script,
               self.build_dir)
...
      log.info("changing mode of %s", file)
...
      log.info("changing mode of %s from %o to %o",
               file, oldmode, newmode)

What I don't see is a way to change the log level, but if that were possible then I assume that's how `--quiet` could be achieved here.