ARROW-7868: [Crossbow] Reduce GitHub API query parallelism
ARROW-7868: [Crossbow] Reduce GitHub API query parallelism

We have triggered the abuse detection mechanism of the GitHub API on the dgx1 machine with higher bandwidth https://ci.ursalabs.org/#/builders/99/builds/261

So let set the query parallelism from an environment variable.

Closes #6439 from kszucs/ARROW-7868 and squashes the following commits:

4f5f49c90 <Krisztián Szűcs> limit query parallelism

Authored-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

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