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