Fix inadvertently upgrading compiler warnings to errors (dotnet#114323)

I noticed this while looking at an issue in the 9.0 branch with latest clang which runs into a couple warnings fixed in main with dotnet#108888.

We intentionally don't set `-Werror` in release branches to avoid breaking the build when newer compilers add warnings via https://github.com/dotnet/runtime/blob/37e4d45236e68946db9d264593aa31a9c00534bc/eng/native/configurecompiler.cmake#L564-L567

However this didn't work because msbuild's WarnAsError was still reading the console output of the native build and upgrading any line with "warning: " to an msbuild error and breaking the build.

Suppress this by setting IgnoreStandardErrorWarningFormat="true" on the Exec call. We already did this in the coreclr and mono runtime builds, we just missed it in libs.native.

Author: akoeplinger

src/native/libs/build-native.proj

@@ -70,7 +70,7 @@
 
     <Copy SourceFiles="@(_IcuArtifacts)" DestinationFolder="$([MSBuild]::NormalizeDirectory('$(ArtifactsBinDir)', 'native', '$(_BuildNativeOutConfig)'))" SkipUnchangedFiles="true" />
     <Message Text="$(MSBuildThisFileDirectory)build-native.sh $(_BuildNativeUnixArgs)" Importance="High"/>
-    <Exec Command="&quot;$(MSBuildThisFileDirectory)build-native.sh&quot; $(_BuildNativeUnixArgs)" EnvironmentVariables="$(_BuildNativeEnvironmentVariables)" />
+    <Exec Command="&quot;$(MSBuildThisFileDirectory)build-native.sh&quot; $(_BuildNativeUnixArgs)" EnvironmentVariables="$(_BuildNativeEnvironmentVariables)" IgnoreStandardErrorWarningFormat="true" />
   </Target>
 
   <Target Name="BuildNativeWindows"
@@ -82,7 +82,7 @@
     </PropertyGroup>
     <!-- Run script that uses CMake to generate and build the native files. -->
     <Message Text="&quot;$(MSBuildThisFileDirectory)build-native.cmd&quot; $(_BuildNativeArgs)" Importance="High"/>
-    <Exec Command="&quot;$(MSBuildThisFileDirectory)build-native.cmd&quot; $(_BuildNativeArgs)" EnvironmentVariables="$(_BuildNativeEnvironmentVariables)" />
+    <Exec Command="&quot;$(MSBuildThisFileDirectory)build-native.cmd&quot; $(_BuildNativeArgs)" EnvironmentVariables="$(_BuildNativeEnvironmentVariables)" IgnoreStandardErrorWarningFormat="true" />
   </Target>
 
   <UsingTask TaskName="AndroidLibBuilderTask"