/usr/lib/mono/3.5/Microsoft.Build.Tasks.v3.5.dll is in libmono-microsoft-build2.0-cil 2.10.8.1-1ubuntu2.3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/3.5/../gac/Microsoft.Build.Tasks.v3.5/3.5.0.0__b03f5f7f11d50a3a/Microsoft.Build.Tasks.v3.5.dll