Emit ssh UseKeychain option only on macOS

[0xCardinal]

Problem

keychecker passes -o UseKeychain=no to the system ssh client (both in
_run_ssh_command and in the git ls-remote GIT_SSH_COMMAND). UseKeychain
is a macOS-only OpenSSH option. On Linux and other platforms the ssh client
rejects it before doing anything else:

command-line: line 0: Bad configuration option: usekeychain

The ssh process then exits non-zero before authenticating, so keychecker
reports ❌ Authentication failed for every server — even when the key and
account are perfectly valid. This makes the tool effectively unusable for server
validation on Linux (e.g. inside CI, containers, or a pentest jump box).

Reproduction (Linux)

$ keychecker id_rsa --validate huggingface
- huggingface: ❌ Authentication failed
    Server response: command-line: line 0: Bad configuration option: usekeychain

Meanwhile a raw handshake with the same key works fine:

$ ssh -T -i id_rsa -o IdentitiesOnly=yes git@hf.co
Hi <user>, welcome to Hugging Face.

Fix

Gate the UseKeychain option behind a sys.platform == "darwin" check so it is
only emitted on macOS (where IdentitiesOnly=yes is already set to isolate the
key regardless). No behavior change on macOS; Linux/other platforms now complete
the handshake normally.

Testing

Verified end-to-end: after the fix, --validate huggingface correctly resolves
the mapped username on Linux, and macOS behavior is unchanged. Existing test
suite passes and flake8 is clean.