fix: select compatible Python for downstream tests

[wolfv]

Summary

• ignore a downstream package's declared Python test matrix when testing it against a specific upstream artifact
• let the upstream and downstream runtime dependencies select a mutually compatible Python version
• add a regression test for an arch-specific Python 3.10 upstream testing a noarch: python downstream whose own matrix includes Python 3.10 and 3.14

This prevents downstream tests from trying every Python version supported by the downstream package even when some versions are incompatible with the particular upstream artifact under test.

Tests

• cargo check -p rattler_build_core
• cargo test -p rattler_build_core package_test::run_test::tests --lib
• pixi run pytest test/end-to-end/test_simple.py::test_downstream_test -q
• cargo fmt --check