`nvexec::let_value` and friends don't appear to be persisting the function args

[ericniebler]

https://github.com/NVIDIA/stdexec/blob/1d79ecffbc5c80ad9e4a575a3294d84423bda37e/include/nvexec/stream/let_xxx.cuh#LL117-L118

The arguments to let_value's callable should be persisted for the duration of the successor operation. Here, it looks like the arguments are discarded after the callable returns the successor sender.

@senior-zero is there a reason why keeping the arguments around like stdexec::let_value does would be problematic for the CUDA stream scheduler?

[gevtushenko]

@ericniebler I think this was an oversight on my end rather then stream scheduler limitation.