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ARROW-13777: [R] mutate after group_by should be ok as long as there are only scalar functions

Closes #11156 from nealrichardson/mutate-grouped

Lead-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Co-authored-by: Ian Cook <ianmcook@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>