[ci] [R-package] Changed installation strategy to use binaries for data.table

[jameslamb]

Sometimes the R builds on macOS fail, complaining about OpenMP issues in compiling data.table. See #2748 (comment).

Per Matt Dowle's recommendation in Rdatatable/data.table#2406, this PR changes our CI strategy to use the precompiled binaries for data.table available from CRAN. Those tend to only be a week or two between the source distribution, which I think is a reasonable price to pay in exchange for better stability in LightGBM's CI.

For information on the change, see this detail from ?install.packages in R:

[jameslamb]

I see this in the logs:

Warning message:
package ‘data.table’ is not available (as a binary package for R version 3.6.1)

But when I run install.packages('data.table', type='source') on my Mac (running Mojave, and R 3.6.x), it works and I get this in the logs:

trying URL 'https://cran.rstudio.com/bin/macosx/el-capitan/contrib/3.6/data.table_1.12.8.tgz'
Content type 'application/x-gzip' length 2133679 bytes (2.0 MB)
==================================================
downloaded 2.0 MB

I can look into it, but I'm about to start two days of travel. I know this is blocking other PRs so would appreciate if either @Laurae2 can look into it or (worst case) we comment-out the Mac builds and come back and restore them before the next time we do a release.

[StrikerRUS]

I pushed about 15 commits in different branches and didn't see that error anymore. Is it possible that it was a temporary issue with CRAN infrastructure?

[jameslamb]

I pushed about 15 commits in different branches and didn't see that error anymore. Is it possible that it was a temporary issue with CRAN infrastructure?

Yes maybe it was just something transient? Seems to be working on other branches so let's close this PR.