ARROW-8351: [R][CI] Store the Rtools-built Arrow C++ library as a build artifact
ARROW-8351: [R][CI] Store the Rtools-built Arrow C++ library as a build artifact

cc @bkietz @wesm

See build at https://github.com/nealrichardson/arrow/runs/565181788 with artifact attached.

Closes #6852 from nealrichardson/r-win-artifact

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

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