I’ve managed to supress the warning level with /p:WarningLevel=X
msbuild.exe MySolution.sln /t:Rebuild /p:WarningLevel=0 /p:Configuration=Release
^^^^^^^^^^^^^^^^^
Warning
Level Meaning
-------- -------------------------------------------
0 Turns off emission of all warning messages.
1 Displays severe warning messages
2 Displays level 1 warnings plus certain, less-severe warnings, such
as warnings about hiding class members
3 Displays level 2 warnings plus certain, less-severe warnings, such
as warnings about expressions that always evaluate to true or false
4 (the default) Displays all level 3 warnings plus informational warnings