Use the property manager, like phil.o suggested! Define the output path in a file called Directory.build.props. then put this file in a parent folder above all of your project files.
The beauty of this approach is that this MSBuild will automatically import this .props file in each project, if it can find it either in the project directory, or any directory above that location. So you don't even need to change your existing project files! See
Customize your build - MSBuild | Microsoft Docs[
^] for more info.