switch to building multitrait slim

[petrelharp]

In #1840 I switched us to building SLiM from source, and meant to get the "multitrait" feeatures, but missed that I needed to be building it from this branch.

TODO:

• update min slim version (see e.g. test_assert_min_version in test_slim_engine.py)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.62%. Comparing base (f43ddd2) to head (8f961ff).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1844      +/-   ##
==========================================
- Coverage   99.82%   99.62%   -0.20%     
==========================================
  Files         143      143              
  Lines        5030     5030              
  Branches      515      515              
==========================================
- Hits         5021     5011      -10     
- Misses          6       15       +9     
- Partials        3        4       +1     

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