Cache the entire Python environment to speed up build times.

[willgraf]

What

• Cache the entire Python environment in the testing GitHub Action workflow.

Why

• Drastically speed up build times.

[willgraf]

The first build (no cache) with these changes took 10m 2s.

All jobs were restarted (to use the new cache) and which took 3m 34s.

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Dependency installation differences:

• ubuntu: ~0:45 to ~0:10
• windows: ~2:00 to ~0:55
• macOS: ~1:30 to ~1:30 (almost all is restoring the cache)