Fix build breaks and improve perf in multi-targeting projects
When a .NET SDK style msbuild project sets its TargetFrameworks property instead of merely TargetFramework, it builds multiple times. When the resx files need to be updated, each of these target frameworks (that build in parallel) often end up crashing the build because each one runs MAT which tries to rewrite the same files and collides with file locks.
Instead, MAT should only do its work in the cross-targeting build (before the individual target frameworks build). This will reduce its perf impact on the build and avoid such breaks.