ARROW-13082: [CI] Forward R argument to ubuntu-docs build
ARROW-13082: [CI] Forward R argument to ubuntu-docs build

`R=4.1 archery docker run ubuntu-docs`

Closes #10534 from kszucs/forward-r-arg-to-docs-build

Authored-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

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