make socket path configurable

[braun2morrow]

Read the envvar KAMAL_PROXY_SOCKET to determine the control socket path.
This makes it possible to control a running kamal-proxy instance from a
different user, by making the socket accessible via file system
permissions.

If KAMAL_PROXY_SOCKET is unset, retain the previous
${XDG_RUNTIME_DIR:/tmp}/kamal-proxy.sock
behaviour.

[Copilot]

Pull request overview

Adds an environment-variable override for the kamal-proxy control socket path so operators can place the socket in a location with appropriate filesystem permissions (e.g., to allow controlling a running instance from a different user). When KAMAL_PROXY_SOCKET is not set (or is empty), behavior remains the same as before: ${XDG_RUNTIME_DIR:-/tmp}/kamal-proxy.sock.

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Changes:

• Read KAMAL_PROXY_SOCKET to determine the control socket path.
• Preserve the existing default socket path when the env var is not provided.

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