Set SDK_SYSTEMD_SECRET_SOCKET during hook invocation

[tlm]

Description

Adds SDK_SYSTEMD_SECRET_SOCKET to the environment for all SDK hook
invocations, set to the path of the workshop systemd secret socket.
This allows SDK authors to read the socket location from the environment
rather than hard-coding it, so that the path can change over time
without breaking existing SDKs.

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Docs

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