chore(ci): dynamic matrix for reduced private fork build time

[jansegre]

This is what happens on a repo that is not the main repo: jansegre#7

This PR is largely based off https://www.cynkra.com/blog/2020-12-23-dynamic-gha/

Acceptance criteria:

• On the main repo (HathorNetwork/hathor-core), the build matrix should be the complete build matrix, the same as it currently is;
• On any other repo, the build matrix should be reduced (I chose Python-3.7 on Ubuntu);

[codecov]

Codecov Report

Merging #362 (2123d56) into dev (b84d5fb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #362   +/-   ##
=======================================
  Coverage   82.68%   82.68%           
=======================================
  Files         164      164           
  Lines       15084    15084           
  Branches     2143     2143           
=======================================
  Hits        12472    12472           
+ Misses       2182     2181    -1     
- Partials      430      431    +1     

Impacted Files	Coverage Δ
hathor/pubsub.py	89.79% <0.00%> (-4.09%)	⬇️
hathor/consensus.py	95.66% <0.00%> (-0.18%)	⬇️
hathor/p2p/node_sync.py	83.94% <0.00%> (ø)
hathor/p2p/manager.py	69.77% <0.00%> (+1.11%)	⬆️

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