implement Path::getCurrentExecutablePath for Haiku #20189

Job	Run time
build_make (ubuntu:25.10)	4m 58s
build_make (ubuntu:24.04)	4m 34s
build_cmake (alpine:3.23, false, false)	4m 23s
build_make (alpine:3.23)	5m 50s
build_cmake (ubuntu:25.10, true, true)	2m 53s
build_cmake (ubuntu:24.04, true, true)	2m 37s
	25m 15s

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