Fix signal_set::cancel() having no effect when called before wait() #634

Job	Run time
Changelog Summary	54s
FreeBSD 15.0 (Clang): C++20	8m 12s
FreeBSD 14.3 (Clang): C++20	5m 32s
Generate Matrix	5s
GCC 12: C++20	8m 18s
GCC 14: C++20,C++23	4m 36s
GCC 13: C++20,C++23	4m 9s
Clang 20: C++20,C++23 (asan+ubsan, static)	7m 2s
Clang 17: C++20	5m 26s
Clang 17: C++20 (arm64)	3m 47s
GCC 15: C++20,C++23 (tsan)	14m 22s
GCC 14: C++20,C++23 (arm64)	3m 30s
MSVC 14.34: C++20	9m 18s
GCC 15: C++20,C++23	14m 0s
Clang 20: C++20,C++23 (tsan, static)	6m 2s
MSVC 14.42: C++20 (asan)	7m 0s
GCC 15: C++20,C++23 (superproject CMake)	12m 21s
Clang 20: C++20,C++23 (clang-tidy)	5m 20s
GCC 15: C++20,C++23 (coverage, static)	11m 17s
Apple-Clang: C++20,C++23 (macOS 26)	6m 20s
MSVC 14.42: C++20	10m 24s
Clang-CL: C++20	5m 2s
Clang 19: C++20,C++23	5m 25s
Clang 20: C++20,C++23 (arm64)	4m 7s
GCC 13: C++20,C++23 (arm64)	3m 17s
GCC 15: C++20,C++23 (asan+ubsan)	15m 17s
Apple-Clang: C++20,C++23 (macOS 15)	5m 0s
Clang 19: C++20,C++23 (arm64)	4m 1s
Clang-CL: C++20 (asan)	5m 3s
Clang 18: C++20,C++23	5m 20s
Apple-Clang: C++20,C++23 (macOS 26, asan+ubsan)	6m 34s
Apple-Clang: C++20,C++23 (macOS 26, coverage, static)	3m 31s
MinGW Clang: C++20 (static)	9m 43s
Clang 20: C++20,C++23 (x86, static)	7m 16s
Clang 18: C++20,C++23 (arm64)	3m 44s
MinGW Clang: C++20 (coverage, static)	8m 32s
Clang 20: C++20,C++23 (time-trace)	5m 20s
Apple-Clang: C++20,C++23 (macOS 26, tsan, static)	4m 5s
	4h 9m 12s

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