Grpc.Tools add support for env variable GRPC_PROTOC_PLUGIN (fix #27099) (#30411)

* issue 27099 Grpc.Tools add env GRPC_TOOL_PLUGIN
Added environment variable GRPC_TOOL_PLUGIN to override the
GRPC plugin path

* Change GRPC_TOOL_PLUGIN to GRPC_PROTOC_PLUGIN

src/csharp/Grpc.Tools/build/_grpc/_Grpc.Tools.targets

@@ -22,6 +22,10 @@
   <Target Name="gRPC_ResolvePluginFullPath" AfterTargets="Protobuf_ResolvePlatform">
     <PropertyGroup>
       <!-- TODO(kkm): Do not use Protobuf_PackagedToolsPath, roll gRPC's own. -->
+
+      <!-- First try environment variable. -->
+      <gRPC_PluginFullPath Condition=" '$(gRPC_PluginFullPath)' == '' ">$(GRPC_PROTOC_PLUGIN)</gRPC_PluginFullPath>
+
       <gRPC_PluginFullPath Condition=" '$(gRPC_PluginFullPath)' == '' and '$(Protobuf_ToolsOs)' == 'windows' "
            >$(Protobuf_PackagedToolsPath)\$(Protobuf_ToolsOs)_$(Protobuf_ToolsCpu)\$(gRPC_PluginFileName).exe</gRPC_PluginFullPath>
       <gRPC_PluginFullPath Condition=" '$(gRPC_PluginFullPath)' == '' "