[Rd] Built field in binary packages missing the platform

[Gábor Csárdi]

It seems that for binary packages that do not contain compiled code,
the "Built` field looks like this:

Built: R 3.6.0; ; 2019-09-23 13:56:59 UTC; unix

I.e., the second part, the platform string is missing. For a package
that has compiled code, it is present:

Built: R 3.6.0; x86_64-apple-darwin15.6.0; 2019-04-26 19:49:57 UTC;
        unix

This is true for all platforms I tried, Linux, Windows and macOS.

I think it would be great to include the platform in binary packages,
even if they do not have compiled code, because they can still be
platform dependent. Then the Built field could be a simple way to know
what platform a binary package was created for and/or installed on.

Is the platform missing intentionally for some reason?

Thanks,
Gabor