fix: explicitly set Python interpreter in maturin build to prevent using wrong version

[kevinjqliu]

Which issue does this PR close?

• Closes ci: maturin picks up Python 3.8 instead of 3.12 in manylinux container #2276.
• Related to pyiceberg-core 0.9.0 requires building with rust on x86_64 iceberg-python#3190

What changes are included in this PR?

Maturin auto-discovers Python interpreters inside manylinux Docker containers and may pick an older version (e.g., 3.8) instead of the intended 3.12. The actions/setup-python step only installs Python on the host runner, not inside the container. (This is called by in the maturin-action documentation)

Fix by passing -i python3.12 to maturin build in all workflows:

• bindings_python_ci.yml
• release_python.yml
• release_python_nightly.yml

sdist commands are unchanged since source distributions don't target a specific interpreter.

Are these changes tested?

Yes, tested by manually triggering nightly build https://github.com/apache/iceberg-rust/actions/runs/23450629743

Log for wheels (ubuntu-latest, x86_64) shows:

🍹 Building a mixed python/rust project
🔗 Found pyo3 bindings with abi3 support
🐍 Found CPython 3.12 at /opt/python/cp312-cp312/bin/python
📡 Using build options features from pyproject.toml
...
📦 Built wheel for abi3 Python ≥ 3.10 to dist/pyiceberg_core-0.9.0.dev20260323172059-cp310-abi3-manylinux_2_17_x86_64.manylinux2014_x86_64.whl

[Fokko]

Great catch @kevinjqliu

[geruh]

nit: non blocking we can factor out the 3.12 to a variable for future upgrades