ARROW-9151: [R][CI] Fix Rtools 4.0 build: pacman sync
ARROW-9151: [R][CI] Fix Rtools 4.0 build: pacman sync

Closes #7457 from nealrichardson/r-win-pacman-sync

Authored-by: Neal Richardson <beb59f0141269d2d04e73d0dcc1967b64ba87e13@gmail.com>
Signed-off-by: Neal Richardson <beb59f0141269d2d04e73d0dcc1967b64ba87e13@gmail.com>

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