Shorten Unix management REPL test socket path

[conornash]

Hi, I have a few PRs in my repo that I created in order for autolith to build on my Mac. This is the first one. Let me know if it's relevant or useful, or if I should just track my own changes rebased onto your master branch.

The below is the agent explanation of what this PR does:

On macOS the per-user $TMPDIR is long enough that $TMPDIR/management-unix-/private/repl.sock reached 119 bytes, beyond the 104-byte sun_path limit, so test-management-repl-unix-lifecycle failed with "The evaluation Unix socket could not listen".

The test now roots its fixture with test-make-temporary-root, the existing short MKDTEMP helper. Reproduced the failure before the change; 7/7 checks pass after it.

[luciusmagn]

It is absolutely relevant, I am extremely grateful for any and all pull requests :)

I ran to the same issue on Windows (where these things work differently a bit), and fixed it there, had no idea it affected MacOS as well.

I am traveling today, so I won't be able to review and merge your PR until the evening, feel free to submit the others!