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.