Remote store access fails when using SSH multiplexing

[meenzen]

Describe the bug

I have SSH set up to always use ControlMaster / ControlPersist when connecting because I'm using a YubiKey and having to push the Key's button every time is really annoying. With upstream nix this works as expected, but since I've switched to Determinate nix it stopped working. Error:

error: unexpected end-of-file
error: cannot open connection to remote store 'ssh-ng://foo@bar.example': error: protocol mismatch, got 'started

Steps To Reproduce

See #311 which attempts to fix the issue but got closed for some reason.

Expected behavior

nix uses the existing SSH connection and does not fail.

Metadata

Determinate Nixd daemon version: 3.19.0
Determinate Nixd client version: 3.19.0

nix (Determinate Nix 3.19.0) 2.34.6

Additional context

Checklist

• checked latest Determinate Nix manual (source)
• checked open bug issues and pull requests for possible duplicates