Update Sign.proj for central package versions (#39)

Author: jeffkl

Packages.props

@@ -9,6 +9,7 @@
   </ItemDefinitionGroup>
 
   <ItemGroup>
+    <PackageReference Update="MicroBuild.Core" Version="0.3.0" />
     <PackageReference Update="Microsoft.Build" Version="15.7.179" />
     <PackageReference Update="Microsoft.Build.Framework" Version="15.7.179" />
     <PackageReference Update="Microsoft.Build.Locator" Version="1.0.18" />

build/Sign.proj

@@ -13,7 +13,7 @@
     <_GetChildProjectCopyToOutputDirectoryItems>false</_GetChildProjectCopyToOutputDirectoryItems>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="MicroBuild.Core" Version="0.3.0" PrivateAssets="All" />
+    <PackageReference Include="MicroBuild.Core" PrivateAssets="All" />
 
     <ProjectReference Include="..\src\CentralPackageVersions\Microsoft.Build.CentralPackageVersions.csproj"  />
     <ProjectReference Include="..\src\NoTargets\Microsoft.Build.NoTargets.csproj" />