CI: don't cache amici; pytest ignore amici_models; passenv BNGPATH

[dweindl]

• Don't cache the amici installation. We are using amici/develop on purpose to detect problems early on. E.g., #1520 should have been detected much earlier.
• Ignore amici_models when collecting tests
• Use external BNGPATH if set

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.79%. Comparing base (6eb4245) to head (b647f53).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1523 +/- ## =========================================== - Coverage 82.83% 82.79% -0.04% =========================================== Files 163 163 Lines 13962 13962 =========================================== - Hits 11565 11560 -5 - Misses 2397 2402 +5 ```

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[dweindl]

What do we need a BNGPATH for? It is for bionetgen but I am not entirely aware here we use it.

Yes, for BioNetGen, which is used by pysb.

Also used here: https://github.com/ICB-DCM/pyPESTO/blob/3b97e345b63b33297a0dbf7844f68328cc9a378b/test/petab/test_petab_import.py#L23-L29