Csharp fixes

csharp/build_release.sh

@@ -6,9 +6,5 @@ cd $(dirname $(readlink $BASH_SOURCE))
 set DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true
 set DOTNET_CLI_TELEMETRY_OPTOUT=true
 
-# Work around https://github.com/dotnet/core/issues/5881
-dotnet nuget locals all --clear
-
 # Builds Google.Protobuf NuGet packages
-dotnet restore src/Google.Protobuf.sln
-dotnet pack -c Release src/Google.Protobuf.sln -p:ContinuousIntegrationBuild=true
+dotnet pack --no-restore -c Release src/Google.Protobuf.sln -p:ContinuousIntegrationBuild=true

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

@@ -1,4 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <!-- If you update this, update the .csproj in the Docker file as well -->
 
   <PropertyGroup>
     <Description>C# runtime library for Protocol Buffers - Google's data interchange format.</Description>