feat: ILPY_PREFERENCE env var

[tlambert03]

this PR makes it so that you can select the default solver using ILPY_PREFERENCE=scip or ILPY_PREFERENCE=gurobi. this makes it easier to quickly swap backends without changing code. It only changes what happens when a solver is created without an explicit preference, one can still pass preference=Preference.Any etc. If an environment-specified backend is not available, it will cause an error unless Preference.Any is explicitly passed

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 59.15%. Comparing base (0fdfae7) to head (2b49f01). Report is 6 commits behind head on main.

Files	Patch %	Lines
ilpy/wrapper.pyx	50.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #59 +/- ## =========================================== - Coverage 75.72% 59.15% -16.58% =========================================== Files 17 19 +2 Lines 828 1087 +259 Branches 154 306 +152 =========================================== + Hits 627 643 +16 - Misses 136 420 +284 + Partials 65 24 -41 ```

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