Use Codecov carryforward flags

[jdavies-st]

Make the nightly tests flag carryforward so that we can see the coverage of the repo from both unit and nightly tests combined. This should also only compare CI unit tests to previous coverage of CI unit tests, not to coverage of the nightly tests. 🤞

Also, make threshold be 0.1% so that when a few lines of code are deleted, the codecov/project check doesn't fail because the denominator has gotten smaller.

Resolves #4920

[codecov]

Codecov Report

Merging #5406 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5406      +/-   ##
==========================================
+ Coverage   52.39%   52.46%   +0.07%     
==========================================
  Files         414      414              
  Lines       37681    37744      +63     
  Branches     5828     5842      +14     
==========================================
+ Hits        19744    19804      +60     
- Misses      16676    16680       +4     
+ Partials     1261     1260       -1     

Flag	Coverage Δ
#unit	52.46% <ø> (+0.07%)	⬆️

Impacted Files	Coverage Δ
jwst/pipeline/calwebb_image3.py	32.55% <0.00%> (-2.45%)	⬇️
jwst/datamodels/model_base.py	84.56% <0.00%> (-0.89%)	⬇️
jwst/transforms/models.py	86.91% <0.00%> (-0.30%)	⬇️
jwst/extract_1d/extract.py	9.05% <0.00%> (-0.13%)	⬇️
jwst/jump/jump.py	100.00% <0.00%> (ø)
jwst/resample/gwcs_drizzle.py	57.00% <0.00%> (ø)
jwst/extract_1d/extract_1d_step.py	9.09% <0.00%> (ø)
jwst/outlier_detection/outlier_detection_scaled.py	20.51% <0.00%> (ø)
jwst/assign_wcs/util.py	63.17% <0.00%> (+0.31%)	⬆️
jwst/resample/resample_spec_step.py	75.60% <0.00%> (+0.60%)	⬆️
... and 8 more

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[jdavies-st]

Oops, forgot turn turn on coverage reporting for the nightly tests. Done now.