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ARROW-18285: [R] Fix for failing test after lubridate 1.9 release (#14615)

This fixes test failures resulting from an updated lubridate package, whose update removed some functionality that we supported (apparently `lubridate::yq(2021.1)` was a thing). This works in our source because we cast to `string()` before doing any further processing.

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>