Add env-based time limits

[aobolensk]

• read optional env vars PPC_TASK_MAX_TIME and PPC_PERF_MAX_TIME
• use those env vars when checking execution time
• document new environment variables

[github-actions]

Coverage report is available for download
here

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.61%. Comparing base (8b3e8fb) to head (cc89885).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
modules/util/src/util.cpp	50.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
- Coverage   94.57%   93.61%   -0.97%     
==========================================
  Files          11       11              
  Lines         350      360      +10     
  Branches      117      122       +5     
==========================================
+ Hits          331      337       +6     
- Misses          5        7       +2     
- Partials       14       16       +2     

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[allnes]

@aobolensk please write tests and increase coverage

[aobolensk]

Superseded by #525