The installation location for the new MSBuild release has a space in the path.

If I put quotes around the path to MSBuild in system configuration ("C:\Program Files (x86)\MSBuild\12.0\Bin\MSBuild.exe"), this error occurs:

FATAL: "C:\Program Files (x86)\MSBuild\12.0\Bin\MSBuild.exe" doesn't exist
Build step 'Build a Visual Studio project or solution using MSBuild' marked build as failure

Without quotes in the system configuration, but with a command-line parameter that includes quotes (/p:Platform="Mixed Platforms"), this happens:

Path To MSBuild.exe: C:\Program Files (x86)\MSBuild\12.0\Bin\MSBuild.exe
Executing the command cmd.exe /C "C:\Program Files (x86)\MSBuild\12.0\Bin\MSBuild.exe" "/p:Platform=Mixed Platforms" src/Build.proj && exit %%ERRORLEVEL%% from C:\jenkins\workspace\wintest
[wintest] $ cmd.exe /C "C:\Program Files (x86)\MSBuild\12.0\Bin\MSBuild.exe" "/p:Platform=Mixed Platforms" src/Build.proj && exit %%ERRORLEVEL%%
'C:\Program' is not recognized as an internal or external command,
operable program or batch file.
Build step 'Build a Visual Studio project or solution using MSBuild' marked build as failure