CMakeLists: remove/cleanup passing of CMAKE_SYSTEM_NAME (#10351)

Just set it from `${CMAKE_HOST_SYSTEM_NAME}` directly, instead of passing it
from the main CMake file (CMAKE_SYSTEM_NAME defaults to it, but is empty
in script mode).

Initially added in 9ce81f7b2, but then even used with unrelated commands
(that do not use RunTests.cmake, e.g. 221f6fffa).

CMakeLists.txt

@@ -602,7 +602,6 @@ if(BUSTED_PRG)
         -DTEST_DIR=${CMAKE_CURRENT_SOURCE_DIR}/test
         -DBUILD_DIR=${CMAKE_BINARY_DIR}
         -DTEST_TYPE=unit
-        -DSYSTEM_NAME=${CMAKE_SYSTEM_NAME}
         -P ${PROJECT_SOURCE_DIR}/cmake/RunTests.cmake
       DEPENDS ${UNITTEST_PREREQS}
       ${TEST_TARGET_ARGS})
@@ -633,7 +632,6 @@ if(BUSTED_PRG)
       -DTEST_DIR=${CMAKE_CURRENT_SOURCE_DIR}/test
       -DBUILD_DIR=${CMAKE_BINARY_DIR}
       -DTEST_TYPE=functional
-      -DSYSTEM_NAME=${CMAKE_SYSTEM_NAME}
       -P ${PROJECT_SOURCE_DIR}/cmake/RunTests.cmake
     DEPENDS ${FUNCTIONALTEST_PREREQS}
     ${TEST_TARGET_ARGS})
@@ -650,7 +648,6 @@ if(BUSTED_PRG)
       -DTEST_DIR=${CMAKE_CURRENT_SOURCE_DIR}/test
       -DBUILD_DIR=${CMAKE_BINARY_DIR}
       -DTEST_TYPE=benchmark
-      -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
       -P ${PROJECT_SOURCE_DIR}/cmake/RunTests.cmake
     DEPENDS ${BENCHMARK_PREREQS}
     ${TEST_TARGET_ARGS})
@@ -668,7 +665,6 @@ if(BUSTED_LUA_PRG)
       -DTEST_DIR=${CMAKE_CURRENT_SOURCE_DIR}/test
       -DBUILD_DIR=${CMAKE_BINARY_DIR}
       -DTEST_TYPE=functional
-      -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
       -P ${PROJECT_SOURCE_DIR}/cmake/RunTests.cmake
     DEPENDS ${FUNCTIONALTEST_PREREQS}
     ${TEST_TARGET_ARGS})
@@ -681,7 +677,6 @@ if(LUACHECK_PRG)
       -DLUACHECK_PRG=${LUACHECK_PRG}
       -DLUAFILES_DIR=${CMAKE_CURRENT_SOURCE_DIR}/test
       -DIGNORE_PATTERN="*/preload.lua"
-      -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
       -P ${PROJECT_SOURCE_DIR}/cmake/RunLuacheck.cmake)
 
   add_custom_target(
@@ -690,7 +685,6 @@ if(LUACHECK_PRG)
       ${CMAKE_COMMAND}
         -DLUACHECK_PRG=${LUACHECK_PRG}
         -DLUAFILES_DIR=${CMAKE_CURRENT_SOURCE_DIR}/src/nvim/lua
-        -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
         -DREAD_GLOBALS=vim
         -P ${PROJECT_SOURCE_DIR}/cmake/RunLuacheck.cmake
   )
@@ -700,7 +694,6 @@ if(LUACHECK_PRG)
       ${CMAKE_COMMAND}
         -DLUACHECK_PRG=${LUACHECK_PRG}
         -DLUAFILES_DIR=${CMAKE_CURRENT_SOURCE_DIR}/runtime/lua
-        -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
         -DREAD_GLOBALS=vim
         -P ${PROJECT_SOURCE_DIR}/cmake/RunLuacheck.cmake
   )

cmake/RunTests.cmake

@@ -41,7 +41,7 @@ endif()
 set(ENV{TMPDIR} "${BUILD_DIR}/Xtest_tmpdir/${rel_test_path}")
 execute_process(COMMAND ${CMAKE_COMMAND} -E make_directory $ENV{TMPDIR})
 
-set(ENV{SYSTEM_NAME} ${SYSTEM_NAME})
+set(ENV{SYSTEM_NAME} ${CMAKE_HOST_SYSTEM_NAME})  # used by test/helpers.lua.
 execute_process(
   COMMAND ${BUSTED_PRG} -v -o ${BUSTED_OUTPUT_TYPE}
     --lazy --helper=${TEST_DIR}/${TEST_TYPE}/preload.lua