CreateDirectory: use /sbin/mount on macOS

[cole-h]

Some users report mount not being found in PATH when using
Terminal.app (even though I can't reproduce this).

Description

Closes #1021.
Closes #1160.

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)

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