ssh-agent: add forceOverride option to override system SSH_AUTH_SOCK

[XYenon]

Description

On macOS, the system ssh-agent sets SSH_AUTH_SOCK by default, which prevents Home Manager's ssh-agent from being used. This adds a forceOverride option (default true on macOS, false on Linux) that unconditionally sets SSH_AUTH_SOCK to override system settings.

Checklist

• Change is backwards compatible.

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