Change how UpdateXlfOnBuild is done for PRs

[jsoref]

It'd be really nice (especially for people who can't manage to get dotnet 8 working on their arm macOS systems...) if for PRs, CI would in one place run with UpdateXlfOnBuild=true, perform a diff (showing the output in the log) and add a failure if there's a diff (so that it's still a failure condition).

The end result of this would be that if some hapless soul (like me) can't get msbuild /t:UpdateXlf to work, the CI would still fail, but it'd also provide a patch which could be applied to fix up the xlf files and thus fix up the PR so that they could continue.

[vzarytovskii]

You can run (in your PR) /run xlf, this should generate files and create a commit. I have never tried it though, should work (update from last main, I fixed a type there).

[jsoref]

Thanks. It'd be nice if the error message suggested that.