declare that `stdexec::task` is already scheduler-affine #2839

Job	Run time
macos-CPU (MacOS) (clang, Release)	4m 57s
macos-CPU (MacOS) (clang, Debug)	3m 54s
CPU (Windows) (msvc, Release)	14m 50s
CPU (clang 16, Debug)	4m 16s
CPU (gcc 11, Debug)	4m 13s
CPU (Windows) (msvc, Debug)	18s
CPU (gcc 11, Release)	3m 29s
CPU (clang 16, Release)	4m 13s
CPU (clang 16, Debug, c++23)	4m 8s
CPU (gcc 12, Release, TSAN)	4m 13s
CPU (gcc 11, Release, ASAN)	12m 56s
CPU (gcc 14, Release, c++23)	4m 23s
CPU (gcc 13, Debug)	4m 3s
CPU (clang 16, Release, ASAN)	4m 19s
CPU (gcc 14, Release, LEAK)	4m 18s
CI (CPU) (MacOS)	4s
CI (CPU)	3s
CI (CPU) (Windows)	-1s
CPU (Windows) (msvc, Debug)	12m 25s
macos-CPU (MacOS) (clang, Release)	4m 57s
CPU (gcc 11, Debug)	4m 13s
macos-CPU (MacOS) (clang, Debug)	3m 54s
CI (CPU) (MacOS)	4s
CPU (gcc 11, Release, ASAN)	12m 56s
CPU (gcc 14, Release, LEAK)	4m 18s
CPU (gcc 11, Release)	3m 29s
CPU (clang 16, Release, ASAN)	4m 19s
CPU (gcc 12, Release, TSAN)	4m 13s
CPU (gcc 13, Debug)	4m 3s
CPU (gcc 14, Release, c++23)	4m 23s
CI (CPU)	3s
CPU (clang 16, Debug, c++23)	4m 8s
CPU (Windows) (msvc, Release)	14m 50s
CPU (clang 16, Debug)	4m 16s
CPU (clang 16, Release)	4m 13s
CI (CPU) (Windows)	9s
	2h 49m 29s

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