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ARROW-8604: [R][CI] Update CI to use R 4.0

Among the updates and fixes here:

* Change the default R version everywhere from 3.6 to 4.0 (current version as of April 24, 2020). Some jobs that were using a "release" alias were already running on 4.0, but many were not.
* Adapt the apt dockerfile to find the 4.0 CRAN apt servers
* Switch the Windows builds to use the new Rtools40 toolchain, which includes enough msys2 functionality that we can delete a bunch of setup
* Fix a few bugs in the crossbow config that meant that we weren't actually testing on a build matrix of linux distros (environment variables needed to be exported to get picked up by docker-compose.

Closes #7107 from nealrichardson/r-4

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