[R] reformat build scripts

[jameslamb]

Proposes some small changes to the build scripts in the R package, with to make them a bit easier to read (in my opinion) and to make diffs slightly easier to review (via breaking long blocks of arguments out into one argument per line).

this PR is only moving code around and should not change behavior.

I'm sorry for a style-only PR, but I do really think this will make it a bit easier for contributors to understand how the R package is built. If you'd rather not review such style-only PRs, please let me know and I'll stop submitting things like this.

Notes for Reviewers

We've been using similar formatting in {lightgbm} for a few years without issue, e.g. https://github.com/microsoft/LightGBM/blob/1881a501782e0ab1e7618b3ab6ff8871ee311860/R-package/src/Makevars.win.in#L18-L22.

In that project, we also use cmakelint to standardize formatting in CMakeLists.txt and *.cmake scripts... do you have any interest in adopting that tool here in XGBoost? If so I'd be happy to put up a separate PR proposing its use in CI here.

Thanks for your time and consideration.

[trivialfis]

Thank you for cleaning up the code!

but I do really think this will make it a bit easier for contributors to understand how the R package is built

I agree. Small changes add up. Thank you for working on the R package.

n that project, we also use cmakelint to standardize formatting in CMakeLists.txt and *.cmake scripts

That sounds awesome!