Use GitHub mirror to download GNU parallel

[tautschnig]

The host at savannah.gnu.org appears to be down more often than not.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.37%. Comparing base (5dc709d) to head (e84d0a3).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8631   +/-   ##
========================================
  Coverage    80.37%   80.37%           
========================================
  Files         1686     1686           
  Lines       206872   206872           
  Branches        73       73           
========================================
  Hits        166265   166265           
  Misses       40607    40607           

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