[OSX catalina] install script fails if a volume starting with [nN]ix.* already exists

[johnalotoski]

Describe the bug

• The nix catalina install script will fail if a user happens to have a volume already named with a regex pattern of [nN]ix.* since the find_nix_volume() function of the create-darwin-volume.sh script uses this pattern match to determine if a nix store has already been created.

Steps To Reproduce

• Install catalina with a fresh drive, APFS, case-sensitive (as an example). Name the volume something like: nixify-me.
• Once installed, try installing nix:

sh <(curl -L https://nixos.org/nix/install) --darwin-use-unencrypted-nix-store-volume --daemon

• It will fail because it sees nixify-me and believes the appropriate volume for the nix-store has already been created.

Expected behavior

• The install to work.

Additional context

• Ironically, my behavior of naming the Catalina block device volume for what I intended to do with it unwittingly caused the failure :).
• So others don't stumble upon the same fail, it may be good to:
  • Make the regex match more stringent. For example, the script creates a volume having a label starting with Nix Store, so why not pattern match on that?
  • Add a line or two in the manual detailing what the volume name will be if created, and what it is looking for to check for if it already exists.