Reduce the number of concurrent testruns in cibuild

[bkoelman]

Closes #1241.

I've run the AppVeyor cibuild of this PR 10 times, spread throughout the day.
They were all green each time, so this seems to fix the issue.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• N/A: Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Merging #1247 (7c5e7cd) into master (be99234) will not change coverage.
The diff coverage is n/a.

❗ Current head 7c5e7cd differs from pull request most recent head 9b6fac9. Consider uploading reports for the commit 9b6fac9 to get more accurate results

@@           Coverage Diff           @@
##           master    #1247   +/-   ##
=======================================
  Coverage   92.62%   92.62%           
=======================================
  Files         243      243           
  Lines        7795     7795           
=======================================
  Hits         7220     7220           
  Misses        575      575           

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