/usr/lib/mono/xbuild/12.0/ is in mono-xbuild 4.2.1.102+dfsg2-7ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/mono/xbuild/12.0/bin/
/usr/lib/mono/xbuild/12.0/bin/MSBuild/
/usr/lib/mono/xbuild/12.0/bin/MSBuild/Microsoft.Build.CommonTypes.xsd
/usr/lib/mono/xbuild/12.0/bin/MSBuild/Microsoft.Build.Core.xsd
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Build.Engine.dll
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Build.Framework.dll
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Build.Tasks.v12.0.dll
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Build.Utilities.v12.0.dll
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Build.dll
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Build.xsd
/usr/lib/mono/xbuild/12.0/bin/Microsoft.CSharp.targets
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Common.targets
/usr/lib/mono/xbuild/12.0/bin/Microsoft.Common.tasks
/usr/lib/mono/xbuild/12.0/bin/Microsoft.VisualBasic.targets
/usr/lib/mono/xbuild/12.0/bin/Mono.XBuild.Tasks.dll
/usr/lib/mono/xbuild/12.0/bin/xbuild.exe
/usr/lib/mono/xbuild/12.0/bin/xbuild.exe.config
/usr/lib/mono/xbuild/12.0/bin/xbuild.rsp