Should a `let_value` sender be copyable if the sender returned by the `let_value` callable is copyable

[msimberg]

E.g. https://github.com/eth-cscs/DLA-Future/blob/6518755e2f44838b75a9b130c8f3c249f6847b34/include/dlaf/matrix/tile.h#L434-L437. In the example the value sent by the input sender is not copyable, the value sent by the sender returned by the let_value callable is copyable (through async_rw_mutex), but the sender from let_value seems to not be copyable. Is that a fundamental constraint or can that be lifted?