Use a UUID instead of volume name for fstab on Mac

[Hoverbear]

Description

Fixes #212.

Before we were using a volume name for the /etc/fstab entry for the Nix Store, while this worked on our testing Macs, it was insufficiently robust for "Real" Macs.

This introduces a new Darwin-only action called CreateFstabEntry which inquires with diskutil info ${LABEL} for a UUID and then sets that in the /etc/fstab.

Unfortunately adding a test for this is rather hard since we can't reboot CI macs.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[cole-h]

That aside, LGTM (but I don't have a Mac to comprehensively test this on, so maybe we should wait for someone who does?)