[CI] Forward R argument to ubuntu-docs build

[asfimport]

The R version set from environment variable is not propagated to the linux-apt-docs.dockerfile

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #10534

_{Note: This issue was originally created as ARROW-13082. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 10534
#10534