Tests: Disable CMAKE_SYSTEM_PROGRAM_PATH in IgnorePrefixPath test

Some systems set CMAKE_SYSTEM_PROGRAM_PATH, which pollutes the
environment for this test. Erase it before executing the test to get
a clean environment.

Fixes: #23300

Tests/RunCMake/find_program/IgnorePrefixPath.cmake

@@ -5,6 +5,7 @@ function(assert_eq var value)
 endfunction()
 
 set(CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH FALSE)
+set(CMAKE_SYSTEM_PROGRAM_PATH)
 
 set(CMAKE_PREFIX_PATH ${CMAKE_SOURCE_DIR}/Prefix)
 set(_old_CMAKE_SYSTEM_PREFIX_PATH ${CMAKE_SYSTEM_PREFIX_PATH})