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 81.16%. Comparing base (5f9a8d6) to head (bc9be38).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1344      +/-   ##
==========================================
+ Coverage   80.90%   81.16%   +0.25%     
==========================================
  Files          61       62       +1     
  Lines        4903     4974      +71     
==========================================
+ Hits         3967     4037      +70     
- Misses        936      937       +1     

Files with missing lines	Coverage Δ
src/setup.jl	97.63% <100.00%> (ø)

... and 2 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...