Hello!

I have made some improvements to make_custom_pi_os after having some friction getting started on macos:

Improve portability of make_custom_pi_os

• Use which to check for gsed and use it if available:
  macos ships with BSD sed which doesn't support -i for in-place
  editing
• get the primary group of the current user with id -gn instead
  of hardcoding it to ${USER} - macos doesn't create a group
  with the same name as the user by default
• Add quotes around a bunch of variables to prevent globbing /
  word splitting
• Use set -euo pipefail to make the script fail on errors and
  undefined variables, rather than silently continuing
• Use '-s' flag with curl to prevent it from printing the download
  progress bar when retrieving version info
• Check we can retrieve the filename of the latest Raspbian image
  before downloading it
• Print out the URL of the image we're downloading

Allow specifying variant when creating new custom os

Rather than just the hardcoded raspios_lite_armhf, users can now specify
raspios_lite_armhf or raspios_lite_arm64 with -v/--variant

Guard against space / hyphen in the destination folder

This causes some pain, as the default module uses this directory name for creating variables.
If these characters are just disallowed, this pain goes away