(#571) Build: enable the nullable warnings back

Can't remember why I wanted to disable them, to be honest.

Author: ForNeVeR

build/_build.csproj

@@ -10,7 +10,7 @@ SPDX-License-Identifier: MIT
         <OutputType>Exe</OutputType>
         <TargetFramework>net9.0</TargetFramework>
         <RootNamespace></RootNamespace>
-        <NoWarn>CS0649;CS0169;nullable</NoWarn>
+        <NoWarn>CS0649;CS0169</NoWarn>
         <NukeRootDirectory>..</NukeRootDirectory>
         <NukeScriptDirectory>..</NukeScriptDirectory>
         <NukeTelemetryVersion>1</NukeTelemetryVersion>