Fix race in POSIX port `vPortEndScheduler` #2948

Job	Run time
GNU ARM Toolchain	1m 40s
CMake Example with Native GCC	27s
GCC MicroBlaze Toolchain	4m 0s
GNU MSP430 Toolchain	1m 7s
Native GCC (macos-latest)	2m 26s
Native GCC (ubuntu-latest)	1m 48s
WIN32 MingW	2m 31s
WIN32 MSVC	3m 25s
	17m 24s

Provide feedback