ci: add c++ coverage

[tlambert03]

trying to add gcov coverage of the c++ code to the codecov reports here

[codecov]

Codecov Report

Merging #45 (4fc1056) into main (d1fb38d) will decrease coverage by 26.18%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main      #45       +/-   ##
===========================================
- Coverage   91.90%   65.72%   -26.18%     
===========================================
  Files           4       17       +13     
  Lines         383      817      +434     
  Branches        0      151      +151     
===========================================
+ Hits          352      537      +185     
- Misses         31      221      +190     
- Partials        0       59       +59     

see 14 files with indirect coverage changes

[tlambert03]

nice! it works!

[tlambert03]

coverage takes a big hit here... but it's more honest :)

unfortunately, the gurobi license doesn't seem to be working anymore, so we're not getting any gurobi backend coverage. will try to fix in a followup

edit: scratch that, they do work, but not on PRs from forks. So, the coverage works on main