use base64 string as argument to workflow call

[elfkuzco]

Created openzim/overview#66 which now accepts offliner defintion as base64 input. Should be merged before this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.28%. Comparing base (9893ba0) to head (9eb2573).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #48   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files           4        4           
  Lines         376      376           
  Branches       37       37           
=======================================
  Hits          347      347           
  Misses         25       25           
  Partials        4        4           

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