ci: Use conda with python 3.8 (defaults to 3.10 otherwise)

[FabioLuporini]

No description provided.

[codecov]

Codecov Report

Merging #1775 (44e2b5e) into master (b714d55) will decrease coverage by 3.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1775      +/-   ##
==========================================
- Coverage   89.47%   86.44%   -3.03%     
==========================================
  Files         208      203       -5     
  Lines       33932    33385     -547     
  Branches     4408     4375      -33     
==========================================
- Hits        30361    28861    -1500     
- Misses       3074     4008     +934     
- Partials      497      516      +19     

Impacted Files	Coverage Δ
tests/test_gpu_openacc.py	5.80% <0.00%> (-92.26%)	⬇️
tests/test_adjoint.py	26.66% <0.00%> (-73.34%)	⬇️
devito/ir/iet/efunc.py	40.62% <0.00%> (-50.00%)	⬇️
examples/seismic/tti/operators.py	53.94% <0.00%> (-43.16%)	⬇️
devito/passes/iet/orchestration.py	18.07% <0.00%> (-41.81%)	⬇️
examples/seismic/self_adjoint/operators.py	58.46% <0.00%> (-41.54%)	⬇️
examples/seismic/viscoacoustic/operators.py	59.49% <0.00%> (-40.51%)	⬇️
examples/seismic/tti/wavesolver.py	68.59% <0.00%> (-29.76%)	⬇️
examples/seismic/elastic/elastic_example.py	48.57% <0.00%> (-28.58%)	⬇️
...mples/seismic/viscoelastic/viscoelastic_example.py	48.57% <0.00%> (-28.58%)	⬇️
... and 48 more

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