Fix incremental build of _RestoreBuildToolsPackagesConfig target when target-specific packages file doesn't exist
Fix incremental build of _RestoreBuildToolsPackagesConfig target when target-specific packages file doesn't exist

mono/msbuild,rainersigwald/msbuild,Microsoft/msbuild,cdmihai/msbuild,jeffkl/msbuild,jeffkl/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,jeffkl/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,mono/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,cdmihai/msbuild,chipitsine/msbuild,mono/msbuild,cdmihai/msbuild,mono/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,cdmihai/msbuild,chipitsine/msbuild,jeffkl/msbuild,Microsoft/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild