Update `pthreadpool` to use the Google-maintained fork at https://git… #4176

Job	Run time
build-and-test / cmake-linux-aarch64	7m 37s
build-and-test / cmake-android (arm64)	7m 58s
build-and-test / cmake-android (armv7)	6m 52s
build-and-test / cmake-linux-armhf	6m 44s
build-and-test / cmake-android (x86)	8m 57s
build-and-test / cmake-linux-riscv64	7m 48s
build-and-test / cmake-linux-riscv64-rvv	13m 17s
build-and-test / cmake-linux-x86_64	9m 59s
build-and-test / cmake-macos-arm64	11m 29s
build-and-test / cmake-windows-arm64	2m 48s
build-and-test / cmake-windows-x64	6m 15s
build-and-test / cmake-windows-x86	7m 45s
build-and-test / bazel-linux-aarch64-clang18	8m 48s
build-and-test / bazel-linux-aarch64-gcc13	7m 21s
build-and-test / bazel-linux-x86_64-clang-18	11m 12s
build-and-test / bazel-linux-x86_64-gcc-9	6m 57s
build-and-test / cmake-ios-arm64	2m 59s
build-and-test / cmake-ios-x86_64	5m 54s
	2h 20m 40s

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