CI: enable coverage tracking in generated gap.sh

[fingolfin]

... so that we also see code coverage for the Julia code in JuliaExperimental.

At least that's the theory. Let's see if it works out.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (3eb636d) to head (df42358).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1344      +/-   ##
==========================================
+ Coverage   78.03%   78.77%   +0.73%     
==========================================
  Files          61       61              
  Lines        4945     4947       +2     
==========================================
+ Hits         3859     3897      +38     
+ Misses       1086     1050      -36     

Files with missing lines	Coverage Δ
src/setup.jl	90.97% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[fingolfin]

Nope, that's not it or at least that's not enough. Trying some more stuff...