Correct platform-specific installs in CI, and add codecov tags

[ascillitoe]

We were not actually running some ops dep-dependent tests (keops and fbprophet) in CI due to some syntax issues in the github action yaml's. This PR fixes this.

Codecov flags are also added so that we can view coverage on a per-build basis.

[codecov-commenter]

Codecov Report

Merging #615 (52a4282) into master (571d501) will decrease coverage by 5.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
- Coverage   83.58%   77.96%   -5.63%     
==========================================
  Files         207      123      -84     
  Lines       13840     8749    -5091     
==========================================
- Hits        11568     6821    -4747     
+ Misses       2272     1928     -344     

Flag	Coverage Δ
ubuntu-latest-3.10	77.34% <ø> (?)
ubuntu-latest-3.7	77.75% <ø> (?)
ubuntu-latest-3.8	77.79% <ø> (?)
ubuntu-latest-3.9	77.79% <ø> (?)
windows-latest-3.9	74.47% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
alibi_detect/models/tensorflow/embedding.py	30.76% <0.00%> (-57.70%)	⬇️
alibi_detect/utils/prediction.py	75.00% <0.00%> (-25.00%)	⬇️
alibi_detect/saving/saving.py	78.19% <0.00%> (-8.06%)	⬇️
alibi_detect/saving/loading.py	82.88% <0.00%> (-6.96%)	⬇️
alibi_detect/utils/tensorflow/prediction.py	88.23% <0.00%> (-5.89%)	⬇️
alibi_detect/saving/tensorflow/_saving.py	75.52% <0.00%> (-5.68%)	⬇️
alibi_detect/cd/tensorflow/preprocess.py	89.79% <0.00%> (-4.09%)	⬇️
alibi_detect/datasets.py	68.69% <0.00%> (-0.64%)	⬇️
alibi_detect/saving/tensorflow/_loading.py	83.89% <0.00%> (-0.29%)	⬇️
alibi_detect/cd/learned_kernel.py	100.00% <0.00%> (ø)
... and 94 more

[ascillitoe]

Note: Once merged we should add tags and remove --cov-append for alibi (or keep, depending on final decision).