CMakeDetermineSystem: Do not try to lookup 'uname' in CMAKE_SYSROOT

We find `uname` before the toolchain file has been included, so the user
has not yet been able to set `CMAKE_FIND_ROOT_PATH_MODE_PROGRAM`.  If
`CMAKE_SYSROOT` is passed via the command line, avoid searching in it.

Modules/CMakeDetermineSystem.cmake

@@ -9,7 +9,7 @@
 
 # find out on which system cmake runs
 if(CMAKE_HOST_UNIX)
-  find_program(CMAKE_UNAME NAMES uname PATHS /bin /usr/bin /usr/local/bin)
+  find_program(CMAKE_UNAME NAMES uname PATHS /bin /usr/bin /usr/local/bin NO_CMAKE_FIND_ROOT_PATH)
   if(CMAKE_UNAME)
     if(CMAKE_HOST_SYSTEM_NAME STREQUAL "AIX")
       execute_process(COMMAND ${CMAKE_UNAME} -v