fix random_string #1889

Job	Run time
build (macos-14, armv8-clang-14)	7m 15s
build-test-downstream (ubuntu-24.04, clang-18)	3m 55s
build (ubuntu-24.04, gcc-14)	7m 54s
build (macos-13, clang-14)	7m 36s
build-test-downstream (macos-13, clang-14)	2m 53s
build-test-downstream (ubuntu-24.04, gcc-14)	3m 55s
build (windows-2022, msvc-1940)	4m 43s
build (ubuntu-24.04, clang-18)	8m 17s
build-test-downstream (windows-2022, msvc-1940)	5m 2s
build-test-downstream (macos-14, armv8-clang-14)	2m 18s
test (macos-14, armv8-clang-14)	5m 5s
docker (ubuntu-24.04, clang-18)	17s
	59m 10s

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