rules_r: export PATH during package build
rules_r: export PATH during package build

Summary:
roxygen 6.1.0 now uses pkgbuild to build the package before generating
documentation. pkgbuild uses `R CMD SHLIB` in a separate processes using
callr.  If PATH is not exported, then compilers may or may not be found
depending on the user's machine.

Reviewers: gmeixiong

Reviewed By: gmeixiong

Maniphest Tasks: T12333

Differential Revision: https://phabricator.grailbio.com/D19922

fbshipit-source-id: ba34080

grailbio/rules_r,grailbio/rules_r,grailbio/rules_r,grailbio/rules_r,grailbio/rules_r