ci: adds fixed env ci

[lkstrp]

Adds pinned env ci and runs test on pinned env instead, as implemented in pypsa-eur.
Removes envs/environment.fixed.yaml and adds envs/<os>-pinned.yaml instead which are automatically updated.
This is useful for the devcontainer implementation by @drifter089 and @GbotemiB

See PyPSA/pypsa-eur#1379

• updates fixed env ci to
  • Create env for each os (ubuntu, windows, linux). They are not compatible across systems.
  • Runs weekly/ on manual trigger/ env changes
• Run tests on fixed env instead
  • This way we don't test dependency changes in the ci and only test the code or if data retrieval fails.
  • Dependency updates are checked in the weekly trigger above.
  • As pypsa-eur is not a library, there is no big reason to keep the dependency constraints wide. And they are still via the normal environment file

[lkstrp]

Tests will fail since the env files need to be created initially. But it looks like they are not running through anyways?

[davide-f]

Many thanks and this seems a great contribution!
This simplifies a lot the maintenance of the model indeed

I'd also recommend adding here a release note also to summarize the great three contributions you proposed :) you can find it in doc/release_notes.rst.
Many thanks :D

[lkstrp]

Thank you and no worries! I added release notes for all three PRs.

The Tests CI fails right now, so this needs another look once this one is merged