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ARROW-10217: [CI] Run fewer GitHub Actions jobs

* Only run the pull-request-labeler if the relevant paths are modified and only on opening/reopening a PR
* Cut 2 R builds from the matrix

Closes #8380 from nealrichardson/fewer-ci-jobs

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>