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

[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 80.90%. Comparing base (5f9a8d6) to head (2e3974b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1343   +/-   ##
=======================================
  Coverage   80.90%   80.90%           
=======================================
  Files          61       61           
  Lines        4903     4903           
=======================================
  Hits         3967     3967           
  Misses        936      936           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[lgoettgens]

This has conflicts now...