ARROW-7416: [R][Nightly] Fix macos-r-autobrew build on R 3.6.2
ARROW-7416: [R][Nightly] Fix macos-r-autobrew build on R 3.6.2

Closes #6053 from nealrichardson/fix-autobrew-3.6.2 and squashes the following commits:

87857e54c <Neal Richardson> Don't run macos-r-autobrew --as-cran

Authored-by: Neal Richardson <beb59f0141269d2d04e73d0dcc1967b64ba87e13@gmail.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>

kou/arrow,laurentgo/arrow,cpcloud/arrow,kou/arrow,apache/arrow,cpcloud/arrow,xhochy/arrow,kou/arrow,wesm/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,kou/arrow,cpcloud/arrow,wesm/arrow,cpcloud/arrow,kou/arrow,laurentgo/arrow,icexelloss/arrow,icexelloss/arrow,wesm/arrow,xhochy/arrow,cpcloud/arrow,kou/arrow,laurentgo/arrow,cpcloud/arrow,icexelloss/arrow,icexelloss/arrow,wesm/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,cpcloud/arrow,laurentgo/arrow,wesm/arrow,xhochy/arrow,icexelloss/arrow,apache/arrow,icexelloss/arrow,apache/arrow,laurentgo/arrow,laurentgo/arrow,xhochy/arrow,laurentgo/arrow,cpcloud/arrow,xhochy/arrow,cpcloud/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,kou/arrow,icexelloss/arrow,wesm/arrow,laurentgo/arrow,xhochy/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,laurentgo/arrow,cpcloud/arrow,wesm/arrow,icexelloss/arrow,xhochy/arrow,wesm/arrow,wesm/arrow,kou/arrow,cpcloud/arrow,wesm/arrow,apache/arrow,kou/arrow,apache/arrow,xhochy/arrow,xhochy/arrow,xhochy/arrow,xhochy/arrow,xhochy/arrow,wesm/arrow,icexelloss/arrow,icexelloss/arrow,wesm/arrow,kou/arrow,icexelloss/arrow,apache/arrow,laurentgo/arrow,apache/arrow,xhochy/arrow,apache/arrow,xhochy/arrow,kou/arrow,xhochy/arrow,laurentgo/arrow,cpcloud/arrow,kou/arrow,wesm/arrow,apache/arrow,cpcloud/arrow,wesm/arrow,apache/arrow,apache/arrow,kou/arrow