ensure no nested parallelism in froll

[jangorecki]

closes #7352

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.13%. Comparing base (59f966c) to head (9197461).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7382   +/-   ##
=======================================
  Coverage   99.12%   99.13%           
=======================================
  Files          85       85           
  Lines       16637    16559   -78     
=======================================
- Hits        16492    16415   -77     
+ Misses        145      144    -1     

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[github-actions]

• HEAD=froll-no-nested-par stopped early for fread disk overhead improved in #6925
  

Generated via commit 9197461

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 45 seconds
Installing different package versions	43 seconds
Running and plotting the test cases	2 minutes and 56 seconds