MacOS installer does not honor case sensitive setting

[Hoverbear]

Description

Fixes #716

> nix-installer install macos --case-sensitive
> sudo diskutil apfs list
....
        ---------------------------------------------------
        APFS Volume Disk (Role):   disk3s7 (No specific role)
        Name:                      Nix Store (Case-sensitive)
        Mount Point:               /nix
        Capacity Consumed:         72744960 B (72.7 MB)
        Sealed:                    No
        FileVault:                 No (Encrypted at rest)

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