ARROW-18220: [Dev] Remove a magic number for the default parallel level in downloader

[kou]

No description provided.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-18220

[github-actions]

⚠️ Ticket has not been started in JIRA, please click 'Start Progress'.

[ursabot]

Benchmark runs are scheduled for baseline = 80ec829 and contender = 6bf5de1. 6bf5de1 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.0% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.93% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 6bf5de17 ec2-t3-xlarge-us-east-2
[Failed] 6bf5de17 test-mac-arm
[Finished] 6bf5de17 ursa-i9-9960x
[Finished] 6bf5de17 ursa-thinkcentre-m75q
[Finished] 80ec8298 ec2-t3-xlarge-us-east-2
[Failed] 80ec8298 test-mac-arm
[Finished] 80ec8298 ursa-i9-9960x
[Finished] 80ec8298 ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java