CI improvements

[dlshriver]

Split ci jobs into system and unit tests to hopefully get some quicker results. Use the cache more aggressively, does not completely reinstall verifiers from scratch if found in the cache.

[codecov]

Codecov Report

Merging #89 (c97cb69) into develop (92aa672) will increase coverage by 0.11%.
The diff coverage is 90.00%.

@@             Coverage Diff             @@
##           develop      #89      +/-   ##
===========================================
+ Coverage    89.64%   89.75%   +0.11%     
===========================================
  Files          138      138              
  Lines         7888     7888              
  Branches      1470     1470              
===========================================
+ Hits          7071     7080       +9     
+ Misses         589      582       -7     
+ Partials       228      226       -2     

Impacted Files	Coverage Δ
dnnv/nn/transformers/simplifiers/squeeze_gemms.py	86.88% <90.00%> (ø)
dnnv/verifiers/mipverify/utils.py	79.38% <0.00%> (-3.06%)	⬇️
dnnv/nn/transformers/simplifiers/squeeze_convs.py	100.00% <0.00%> (+59.09%)	⬆️

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