Restore honoring CMAKE_GENERATOR_INSTANCE in toolchain file

Revert the changes from commit d5b5c19278 (cmGlobalGenerator:
FindMakeProgram() before CMakeDetermineSystem, 2020-06-15,
v3.19.0-rc1~619^2~3) and commit ef91fb02f3 (cmGlobalGenerator:
FindMakeProgram() at a generator-specific time, 2020-11-23,
v3.19.1~2^2).  We must delay selecting the location of MSBuild until
after an instance of Visual Studio has been selected.

It is now safe to revert the ordering because the motivating use
case (sysroot detection in Platform/Android-Determine) has been
implemented another way.

Fixes: #22782

Source/cmGlobalGenerator.cxx

@@ -593,16 +593,6 @@ void cmGlobalGenerator::EnableLanguage(
     }
   }
 
-  if (readCMakeSystem) {
-    // Find the native build tool for this generator.
-    // This has to be done early so that MSBuild can be used to examine the
-    // cross-compilation environment.
-    if (this->GetFindMakeProgramStage() == FindMakeProgramStage::Early &&
-        !this->FindMakeProgram(mf)) {
-      return;
-    }
-  }
-
   //  Load the CMakeDetermineSystem.cmake file and find out
   // what platform we are running on
   if (!mf->GetDefinition("CMAKE_SYSTEM")) {
@@ -676,8 +666,7 @@ void cmGlobalGenerator::EnableLanguage(
     }
 
     // Find the native build tool for this generator.
-    if (this->GetFindMakeProgramStage() == FindMakeProgramStage::Late &&
+    if (!this->FindMakeProgram(mf)) {
-        !this->FindMakeProgram(mf)) {
       return;
     }
   }

Source/cmGlobalGenerator.h

@@ -625,17 +625,6 @@ protected:
 
   std::string GetPredefinedTargetsFolder() const;
 
-  enum class FindMakeProgramStage
-  {
-    Early,
-    Late,
-  };
-
-  virtual FindMakeProgramStage GetFindMakeProgramStage() const
-  {
-    return FindMakeProgramStage::Late;
-  }
-
 private:
   using TargetMap = std::unordered_map<std::string, cmTarget*>;
   using GeneratorTargetMap =

Source/cmGlobalVisualStudioGenerator.h

@@ -167,11 +167,6 @@ protected:
 
   void WriteSLNHeader(std::ostream& fout);
 
-  FindMakeProgramStage GetFindMakeProgramStage() const override
-  {
-    return FindMakeProgramStage::Early;
-  }
-
   bool ComputeTargetDepends() override;
   class VSDependSet : public std::set<std::string>
   {

Source/cmGlobalXCodeGenerator.h

@@ -138,11 +138,6 @@ protected:
   void AddExtraIDETargets() override;
   void Generate() override;
 
-  FindMakeProgramStage GetFindMakeProgramStage() const override
-  {
-    return FindMakeProgramStage::Early;
-  }
-
 private:
   enum EmbedActionFlags
   {