R devel - Jan 2019 - Error in re-building vignettes: '\R' is an unrecognized escape in ... "'D:\temp\R"

This is just an FYI to whoever might look into this - we're now
getting the following on CRAN checks (e.g.
https://cran.r-project.org/web/checks/check_results_future.batchtools.html):

Version: 0.16.1
Check: re-building of vignette outputs
Result: WARN
    Error(s) in re-building vignettes:
    Error: '\R' is an unrecognized escape in character string starting
"'D:\temp\R"
    Execution halted
Flavor: r-devel-windows-ix86+x86_64

using R Under development (unstable) (2019-01-04 r75947)
using platform: x86_64-w64-mingw32 (64-bit)
using session charset: ISO8859-1

I have not attempted to narrow down exactly when this was introduced
(maybe it's related to the recent updates to how vignette are built).

/Henrik