[R][CI] R appveyor job fails due to pacman compression change

[asfimport]

It looks like Appveyor is escaping/shell quoting things it didn't before. This line is failing now: https://github.com/apache/arrow/blob/master/ci/appveyor-build-r.sh#L47

• before (passing): https://ci.appveyor.com/project/ApacheSoftwareFoundation/arrow/builds/30150558/job/48s5uf3750yhvfqf#L2614
• after (failing): https://ci.appveyor.com/project/ApacheSoftwareFoundation/arrow/builds/30166850/job/t4nuy7xqwu8pr4ph#L2632

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #6215

_{Note: This issue was originally created as ARROW-7595. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
The cause seems to be https://www.archlinux.org/news/now-using-zstandard-instead-of-xz-for-package-compression/

The file extensions have changed (along with the compression), so the cp *.xz is failing.

Working on a fix/workaround.

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 6215
#6215