VS: Do not concurrently build custom commands with generated MAIN_DEPENDENCY

Since commit 33c15ae2b9 (VS: Build custom commands concurrently when
possible, 2023-01-19) several tests have failed intermittently with the
VS generator.  It seems that if the `BuildInParallel` setting is
attached to a generated input:

    <CustomBuild Include="generated_input.txt">
      <BuildInParallel Condition="...">true</BuildInParallel>
      <Command Condition="...">copy geneated_input.txt output.txt</Command>
      ...
    </CustomBuild>

then MSBuild does not wait for the input to be generated before running
the command.

This occurs when using `add_custom_command`'s `MAIN_DEPENDENCY`, so
avoid using `BuildInParallel` in that case.

Issue: #18405

Source/cmVisualStudio10TargetGenerator.h

@@ -152,13 +152,18 @@ private:
   void OutputLinkIncremental(Elem& e1, std::string const& configName);
   void WriteCustomRule(Elem& e0, cmSourceFile const* source,
                        cmCustomCommand const& command);
+  enum class BuildInParallel
+  {
+    No,
+    Yes,
+  };
   void WriteCustomRuleCpp(Elem& e2, std::string const& config,
                           std::string const& script,
                           std::string const& additional_inputs,
                           std::string const& outputs,
                           std::string const& comment,
                           cmCustomCommandGenerator const& ccg, bool symbolic,
-                          bool uses_terminal);
+                          BuildInParallel buildInParallel);
   void WriteCustomRuleCSharp(Elem& e0, std::string const& config,
                              std::string const& commandName,
                              std::string const& script,