ensure trailing newline when rewriting conda_build_config.yaml

[minrk]

migrators keep rewriting conda_build_config.yaml with the only change being stripping trailing newlines, like this one

I'm not 100% sure if this is the relevant bit, but I think it is. It is at least clear that this code would produce a file without a trailing newline.

• [x] Pydantic model updated or no update needed

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.06%. Comparing base (3528fe2) to head (628510a).

Files	Patch %	Lines
conda_forge_tick/migrators/migration_yaml.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2696 +/- ## ========================================== - Coverage 75.06% 75.06% -0.01% ========================================== Files 109 109 Lines 11102 11104 +2 ========================================== + Hits 8334 8335 +1 - Misses 2768 2769 +1 ```

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