/usr/lib/mono/xbuild/12.0/bin/Microsoft.Build.dll is in mono-xbuild 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/xbuild/12.0/bin/../gac/Microsoft.Build/12.0.0.0__b03f5f7f11d50a3a/Microsoft.Build.dll