ci: use codecov flags

[flying-sheep]

Seems like env_vars is broken: codecov/codecov-action#1828

/edit: hm, using flags like this seems to also not be right:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.37%. Comparing base (5b1a8c5) to head (65db02d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2217      +/-   ##
==========================================
- Coverage   87.34%   85.37%   -1.97%     
==========================================
  Files          49       49              
  Lines        7775     7775              
==========================================
- Hits         6791     6638     -153     
- Misses        984     1137     +153     

Flag	Coverage Δ
dask_distributed	54.13% <ø> (?)
hatch-test.array_api	56.83% <ø> (?)
hatch-test.min	79.67% <ø> (?)
hatch-test.pre	84.48% <ø> (?)
hatch-test.stable	84.48% <ø> (?)
not zarr_io	84.73% <ø> (?)
zarr_io	67.96% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 8 files with indirect coverage changes