Add WriteOnlyWhenDifferent to SwaggerInitFile target (#1123)

* Prevent writing the swagger init file on every build, keeping the timestamp and preventing dotnet build flagging the user project as changed

* avoid rewriting openapi init file when unchanged

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Co-authored-by: Dĵ ΝιΓΞΗΛψΚ <djnitehawk83@gmail.com>
Co-authored-by: Dĵ ΝιΓΞΗΛψΚ <dj-nitehawk@users.noreply.github.com>