Pin Microsoft.Build dependencies in Statiq.Web.csproj to resolve security issues.

Author: jasonsummers

src/Statiq.Web/Statiq.Web.csproj

@@ -12,6 +12,9 @@
 
   <ItemGroup>
     <PackageReference Include="Buildalyzer.Workspaces" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Build" Version="18.0.2" />
+    <PackageReference Include="Microsoft.Build.Tasks.Core" Version="18.0.2" />
+    <PackageReference Include="Microsoft.Build.Utilities.Core" Version="18.0.2" />
   </ItemGroup>
 
   <ItemGroup>