CI failure on R 3.2

[yutannihilation]

After #4091, R 3.2 runner fails with this warning. It seems cross-referencing to packages that are not on the CRAN index (sf is not available on R 3.2 as it requires R >= 3.3) is not allowed.

❯ checking Rd cross-references ... WARNING
  Packages unavailable to check Rd xrefs: ‘multcomp’, ‘Hmisc’
  Unknown package ‘sf’ in Rd xrefs

We can avoid this by setting _R_CHECK_RD_XREFS_ to FALSE.

https://github.com/wch/r-source/blob/8d7ac4699fba640d030703fa010b66bf26054cbd/src/library/tools/R/check.R#L5988-L5989
https://github.com/wch/r-source/blob/8d7ac4699fba640d030703fa010b66bf26054cbd/src/library/tools/R/check.R#L2187-L2201

c.f. https://cloud.r-project.org/doc/manuals/r-patched/R-ints.html#index-_005fR_005fCHECK_005fRD_005fXREFS_005f