install: fix --no-local-pkgs

As chroot.glob() now returns a generator, convert it to a list first to
check if it is empty. This may not be the most efficient method, but
since we expect it to be empty here it is fine.

Without this patch, it always shows the error even if there are no
locally built packages:

  ERROR: --no-local-pkgs specified, but locally built packages found. Consider 'pmbootstrap zap -p' to delete them.