Fix .NET SourceLink and warnings in NuGet package

Author: JamesNK

csharp/build_packages.bat

@@ -1,7 +1,7 @@
 @rem Builds Google.Protobuf NuGet packages
 
 dotnet restore src/Google.Protobuf.sln
-dotnet pack -c Release src/Google.Protobuf.sln || goto :error
+dotnet pack -c Release src/Google.Protobuf.sln -p:ContinuousIntegrationBuild=true || goto :error
 
 goto :EOF
 

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -15,10 +15,11 @@
     <PackageTags>Protocol;Buffers;Binary;Serialization;Format;Google;proto;proto3</PackageTags>
     <PackageReleaseNotes>C# proto3 support</PackageReleaseNotes>
     <PackageProjectUrl>https://github.com/protocolbuffers/protobuf</PackageProjectUrl>
-    <PackageLicenseUrl>https://github.com/protocolbuffers/protobuf/blob/master/LICENSE</PackageLicenseUrl>
+    <PackageLicenseExpression>BSD-3-Clause</PackageLicenseExpression>
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>https://github.com/protocolbuffers/protobuf.git</RepositoryUrl>
-    <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <EmbedUntrackedSources>true</EmbedUntrackedSources>
     <!-- Include PDB in the built .nupkg -->
     <AllowedOutputExtensionsInPackageBuildOutputFolder>$(AllowedOutputExtensionsInPackageBuildOutputFolder);.pdb</AllowedOutputExtensionsInPackageBuildOutputFolder>
   </PropertyGroup>