MSBuild.exe 15 was found after installing the VS Build Tools with no additional packages selected in the install interface.
Edit: Updated link above to go to older downloads. Expand 2017 panel, click Download, login with a Microsoft account, then you will see Build Tools for Visual Studio 2017 (version 15.9) in the list of available downloads.
Edit: increased minor version to 9 to reflect the latest of major version 15.
Edit: You have to join the free "Dev Essentials" if you do not have a subscription. Then you can proceed to "Downloads" menu item and search "Build Tools for Visual Studio 2017" to get the proper download button.
C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\Bin\MSBuild.exe