CI: pass --coverage to C/LDFLAGS when building JuliaInterface

[fingolfin]

There is a bunch of C code in JuliaInterface that we definitely are exercising, but which Codecov does not report as covered. I am trying to figure out why. One guess is that we only enable the CFLAG --cover when running the gap tests (in etc/ci_test.sh), so this PR tries to enable them also when running the JuliaInterface tests, at least in one case (the "treehash" CI check, as it already enables FORCE_JULIAINTERFACE_COMPILATION=true anyway).

UPDATE: alas, it doesn't work and I am not really sure why

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.94%. Comparing base (3eb636d) to head (c1d9c7c).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1343      +/-   ##
==========================================
+ Coverage   78.03%   78.94%   +0.90%     
==========================================
  Files          61       61              
  Lines        4945     4934      -11     
==========================================
+ Hits         3859     3895      +36     
+ Misses       1086     1039      -47     

see 4 files with indirect coverage changes

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