Ninja Multi-Config: Correctly generate POST_BUILD custom targets

Fixes: #22096

Source/cmNinjaUtilityTargetGenerator.cxx

@@ -173,7 +173,7 @@ void cmNinjaUtilityTargetGenerator::WriteUtilBuildStatements(
     std::string ccConfig;
     if (genTarget->Target->IsPerConfig() &&
         genTarget->GetType() != cmStateEnums::GLOBAL_TARGET) {
-      ccConfig = fileConfig;
+      ccConfig = config;
     }
     if (config == fileConfig ||
         gg->GetPerConfigUtilityTargets().count(genTarget->GetName())) {

Tests/RunCMake/NinjaMultiConfig/CustomCommandOutputGenex-target_post_build-debug-ninja-stdout.txt

+^\[1/2\] target_post_build
+target main build
+\[2/2\] Running utility command for target_post_build
+target post build$

Tests/RunCMake/NinjaMultiConfig/CustomCommandOutputGenex.cmake

@@ -189,3 +189,11 @@ add_custom_target(target_no_cross_byproduct
   COMMAND echo $<CONFIG> target_no_cross_byproduct.txt
   WORKING_DIRECTORY $<CONFIG>
   )
+
+add_custom_target(target_post_build
+  COMMENT target_post_build
+  COMMAND ${CMAKE_COMMAND} -E echo "target main build"
+  )
+add_custom_command(TARGET target_post_build POST_BUILD
+  COMMAND ${CMAKE_COMMAND} -E echo "target post build"
+  )

Tests/RunCMake/NinjaMultiConfig/RunCMakeTest.cmake

@@ -364,6 +364,8 @@ run_ninja(CustomCommandOutputGenex target_no_cross_byproduct-debug build-Debug.n
 run_ninja(CustomCommandOutputGenex clean-debug-graph build-Debug.ninja -t clean)
 run_ninja(CustomCommandOutputGenex target_no_cross_byproduct-debug-in-release-graph build-Release.ninja target_no_cross_byproduct:Debug)
 run_ninja(CustomCommandOutputGenex clean-release-graph build-Release.ninja -t clean)
+# target_post_build
+run_ninja(CustomCommandOutputGenex target_post_build-debug build-Debug.ninja target_post_build)
 unset(RunCMake_TEST_NO_CLEAN)
 
 unset(RunCMake_TEST_BINARY_DIR)