support for pre-catalina macOS?

[abathur]

At least on 10.14, installs fail around volume setup stuff. The canonical installer skips this work (IIRC keying on whether root is read-only or not), so I imagine that's what we'd need to do to support back to the 10.12.6 or whatever Nix currently asserts support for.

Not sure if upstream is amenable to accepting a PR that does this, but we can figure it out here first.

Failures look like:

users-Air% curl -L https://raw.githubusercontent.com/NixOS/experimental-nix-installer/main/nix-installer.sh | sh -s -- install
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 16034  100 16034    0     0   144k      0 --:--:-- --:--:-- --:--:--  146k
info: downloading installer https://raw.githubusercontent.com/NixOS/experimental-nix-installer/prerelease/nix-installer-x86_64-darwin
`nix-installer` needs to run as `root`, attempting to escalate now via `sudo`...
Nix install plan (v0.14.0)
Planner: macos (with default settings)

Planned actions:
* Create an APFS volume `Nix Store` for Nix on `disk1` and add it to `/etc/fstab` mounting on `/nix`
* Fetch `https://releases.nixos.org/nix/nix-2.18.1/nix-2.18.1-x86_64-darwin.tar.xz` to `/nix/temp-install-dir`
* Create a directory tree in `/nix`
* Move the downloaded Nix into `/nix`
* Create build users (UID 300-332) and group (GID 30000)
* Configure Time Machine exclusions
* Setup the default Nix profile
* Place the Nix configuration in `/etc/nix/nix.conf`
* Configure the shell profiles
* Configuring zsh to support using Nix in non-interactive shells
* Create a `launchctl` plist to put Nix into your PATH
* Configure Nix daemon related settings with launchctl
* Remove directory `/nix/temp-install-dir`


Proceed? ([Y]es/[n]o/[e]xplain): y
 INFO Step: Create an APFS volume `Nix Store` for Nix on `disk1` and add it to `/etc/fstab` mounting on `/nix`
ERROR 
   0: Install failure
   1: Error executing action
   2: Action `create_nix_volume` errored
   3: Failed to execute command with status 1 `"/usr/sbin/diskutil" "info" "/nix"`, stdout: Could not find disk: /nix

      stderr: 
   3: 

Location:
   /rustc/d5c2e9c342b358556da91d61ed4133f6f50fc0c3/library/core/src/convert/mod.rs:716

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.

Consider reporting this error using this URL: https://github.com/DeterminateSystems/nix-installer/issues/new?title=%3Cautogenerated-issue%3E&body=%23%23+Error%0A%60%60%60%0AError%3A+%0A+++0%3A+Install+failure%0A+++1%3A+Error+executing+action%0A+++2%3A+Action+%60create_nix_volume%60+errored%0A+++3%3A+Failed+to+execute+command+with+status+1+%60%22%2Fusr%2Fsbin%2Fdiskutil%22+%22info%22+%22%2Fnix%22%60%2C+stdout%3A+Could+not+find+disk%3A+%2Fnix%0A%0A++++++stderr%3A+%0A%0A%60%60%60%0A%0A%23%23+Metadata%0A%7Ckey%7Cvalue%7C%0A%7C--%7C--%7C%0A%7C**version**%7C0.14.0%7C%0A%7C**os**%7Cmacos%7C%0A%7C**arch**%7Cx86_64%7C%0A
Installation failure, offering to revert...
Nix uninstall plan (v0.14.0)

Planner: macos (with default settings)

Planned actions:
* Remove the APFS volume `Nix Store` on `disk1`


Proceed? ([Y]es/[n]o/[e]xplain):