rules_r/razel: fallback to source archives if sha256 is not available
rules_r/razel: fallback to source archives if sha256 is not available

Summary:
For macOS, if package list does not have a column for sha256, do not use binary
archives instead of a hard fail right now.

Reviewers: pknudsgaard

Reviewed By: pknudsgaard

Differential Revision: https://phabricator.grailbio.com/D14730

fbshipit-source-id: 1d8a449

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