fix the attributes of the `let_` senders #3366

Job	Run time
macos-CPU (MacOS) (clang, Debug)	3m 36s
CPU (Windows) (msvc 14.43, Debug)	17m 37s
CPU (Windows) (msvc 14.43, Release)	19m 23s
macos-CPU (MacOS) (clang, Release)	3m 31s
CPU (Windows) (msvc 14.50, Release)	18m 44s
CPU (Windows) (msvc 14.50, Debug)	16m 48s
CPU (gcc 12, Debug)	11m 24s
CPU (gcc 12, Release)	9m 38s
CPU (clang 22, Debug)	9m 35s
CPU (gcc 12, Release, TSAN)	9m 48s
CPU (clang 22, Release)	9m 23s
CPU (clang 16, Debug, TSAN)	6h 5m 0s
CPU (gcc 14, Debug)	10m 43s
CPU (gcc 13, Debug)	10m 29s
CPU (gcc 14, Debug, ASAN)	10m 56s
CPU (clang 16, Release, ASAN)	6h 5m 0s
CPU (clang 16, Debug, c++23)	6h 5m 0s
CPU (clang 16, Debug)	6h 5m 1s
CPU (clang 16, Release)	6h 5m 0s
CPU (gcc 14, Release, ASAN)	9m 40s
CPU (gcc 14, Release, LEAK)	13m 7s
CPU (gcc 15, Release, c++23)	13m 4s
CPU (gcc 14, Debug, TSAN)	10m 23s
CPU (gcc 14, Release, c++23)	9m 58s
CI (CPU) (MacOS)	0s
CI (CPU) (Windows)	8s
CI (CPU)	0s
	1d 10h 2m 56s

Provide feedback