Forward rooted environment in start_detached

[fallintoplace]

Summary

• forward the rooted environment into apply_sender
• add coverage for a move-only environment

Why

The two-argument overload stores the rooted environment in __env2 and then passes that named variable as an lvalue. The default implementation stores the decayed environment by value, so a move-only environment cannot be used. Forwarding __env2 keeps the value category expected by the constraints.

Testing

• cmake --build build --target test.exec -j 8
• ./build/test/exec/test.exec '[start_detached]'

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[ericniebler]

/ok to test 7e930c2