devel/R-cran-data.table: Update to 1.18.6.1

• The test suite requires a valid Sys.timezone(), which fails in poudriere jails as they have no /etc/localtime. Set TZ=UTC via MAKE_ENV, since cran.mk's do-test does not honour TEST_ENV.

ChangeLog: https://cran.r-project.org/web/packages/data.table/news/news.html