[CI] support older bash versions in generate-flavors.sh

[eljohnson92]

What this PR does / why we need it:
this PR adds some more conditionals in generate-flavors.sh to support the older bash versions shipped with MacOS.
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.03%. Comparing base (b6e8121) to head (080a569).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #446   +/-   ##
=======================================
  Coverage   66.03%   66.03%           
=======================================
  Files          76       76           
  Lines        3945     3945           
=======================================
  Hits         2605     2605           
  Misses       1143     1143           
  Partials      197      197           

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