CI: micromamba and environment caching

[VeckoTheGecko]

Migrating to micromamba with environment caching to reduce CI times.

Environment cache key is based on:

• OS
• runner architecture
• python version
• date
• environment file

[VeckoTheGecko]

Well, looks like it only saves a minute (and its the windows tests that are the main slowdown anyway, and those aren't affected).

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Details

Before:

After (cold start):

After (warm start):

[VeckoTheGecko]

Thoughts on this @erikvansebille ? No significant speedup, but I do have a very slight preference for micromamba for CI . I'm happy either way, just want to clear this out of the open PRs :)

[erikvansebille]

Thoughts on this @erikvansebille ? No significant speedup, but I do have a very slight preference for micromamba for CI . I'm happy either way, just want to clear this out of the open PRs :)

I'd be prefer to keep the installation method of our CI as close to that advised in the installation instructions for users/developers, so that our CI also tests installation procedure. So rather stick with conda for now...

[VeckoTheGecko]

Roger that :)