Bump GitVersion.Tool from 5.6.3 to 5.6.4 (#504)

Author: dependabot[bot]

Directory.Build.targets

@@ -11,7 +11,7 @@
         <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
     </ItemGroup>
     <ItemGroup>
-        <PackageReference Update="GitVersion.Tool" Version="5.6.3" />
+        <PackageReference Update="GitVersion.Tool" Version="5.6.4" />
         <PackageReference Update="JetBrains.ReSharper.CommandLineTools" Version="2020.3.2" />
         <PackageReference Update="ReportGenerator" Version="4.8.4" />
         <PackageReference Update="Rocket.Surgery.Nuke" Version="0.14.2" />