Fix NetCore csproj to auto-detect OS (dotnet#555)

Author: cheenamalhotra

src/Microsoft.Data.SqlClient/netcore/src/Microsoft.Data.SqlClient.csproj

@@ -3,7 +3,7 @@
     <AssemblyName>Microsoft.Data.SqlClient</AssemblyName>
     <TargetFrameworks>netcoreapp2.1;netcoreapp3.1;netstandard2.0</TargetFrameworks>
     <GeneratePlatformNotSupportedAssemblyMessage Condition="'$(OSGroup)' == 'AnyOS'">SR.PlatformNotSupported_DataSqlClient</GeneratePlatformNotSupportedAssemblyMessage>
-    <OSGroup Condition="'$(OSGroup)' == ''">Windows_NT</OSGroup>
+    <OSGroup Condition="'$(OSGroup)' == ''">$(OS)</OSGroup>
     <TargetsWindows Condition="'$(OSGroup)'=='Windows_NT'">true</TargetsWindows>
     <TargetsUnix Condition="'$(OSGroup)'=='Unix'">true</TargetsUnix>
     <!-- Allow explicit addition of the Compile files instead of all project files to be included by Default -->

src/Microsoft.Data.SqlClient/tests/DockerLinuxTest/Dockerfile

@@ -12,10 +12,10 @@ ARG PROJFILE=$PROJNAME".csproj"
 ARG DLLFILE=$PROJNAME".dll"
 
 WORKDIR /sqlclient/src/Microsoft.Data.SqlClient/tests/DockerLinuxTest
-RUN dotnet build $PROJFILE -c Release -o /app/build -p:OSGroup=Unix -p:GenerateDocumentationFile=false
+RUN dotnet build $PROJFILE -c Release -o /app/build 
 
 FROM build AS publish
-RUN dotnet publish $PROJFILE -c Release -o /app/publish -p:OSGroup=Unix -p:GenerateDocumentationFile=false
+RUN dotnet publish $PROJFILE -c Release -o /app/publish
 
 FROM base AS final
 WORKDIR /app