Upgrade Microsoft.Build dependencies to 15.9.20 in test project

Author: AArnott

test/Nerdbank.GitVersioning.Tests/Nerdbank.GitVersioning.Tests.csproj

@@ -7,6 +7,7 @@
     <SignAssembly>false</SignAssembly>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
+    <MSBuildPackageVersion>15.9.20</MSBuildPackageVersion>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\..\src\Shared\**\*.cs" LinkBase="Shared" />
@@ -34,8 +35,8 @@
   <ItemGroup>
     <PackageReference Include="coverlet.msbuild" Version="3.1.2" />
     <PackageReference Include="Microsoft.Build.Locator" Version="1.4.1" />
-    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="15.1.548" ExcludeAssets="runtime" />
-    <PackageReference Include="Microsoft.Build" Version="15.1.548" ExcludeAssets="runtime" />
+    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="$(MSBuildPackageVersion)" ExcludeAssets="runtime" />
+    <PackageReference Include="Microsoft.Build" Version="$(MSBuildPackageVersion)" ExcludeAssets="runtime" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.3.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.3.2" />
     <PackageReference Include="Moq" Version="4.18.1" />