Support v1 recipes in CrossPythonMigrator

[mgorny]

Description:

Update CrossPythonMigrator for v1 recipes. Unfortunately, this yields
a bit suboptimal output, as it does not combine conditions:

    - if: build_platform != target_platform
      then: python
    - if: build_platform != target_platform
      then: cross-python_${{ target_platform }}
    - if: build_platform != target_platform
      then: cython

That said, addressing this would increase the divergency between v0
and v1 code in the migrator, and conda-recipe-manager convert suffers
from the same issue. I think a better way forward would be to write
a dedicated v1 recipe migrator that combines adjacent conditions.

Checklist:

• Pydantic model updated or no update needed

Cross-refs, links to issues, etc:

Part of bug #3642

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.04%. Comparing base (141732a) to head (f80d060).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
conda_forge_tick/migrators/cross_compile.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3832      +/-   ##
==========================================
+ Coverage   77.03%   77.04%   +0.01%     
==========================================
  Files         138      138              
  Lines       15394    15406      +12     
==========================================
+ Hits        11859    11870      +11     
- Misses       3535     3536       +1     

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[isuruf]

For v1 recipes shouldn't we be using - if: build_platform != host_platform? We decided on autotools convention of build, host, target

cc @wolfv

[mgorny]

For v1 recipes shouldn't we be using - if: build_platform != host_platform? We decided on autotools convention of build, host, target

cc @wolfv

I see only build_platform and target_platform in https://rattler.build/v0.27.0/selectors/; also in https://conda.org/learn/ceps/cep-0014

[beckermr]

The CEPs we have actually don't define what variables are allowed versus not in the selectors as far as I know.

[mgorny]

Updated this one to use host_platform as well. I'll submit a pull request to update rattler-build docs as well.

[mgorny]

@beckermr, gentle ping. Is there anything left blocking this PR?