cartopy: add build-cartopy.yml for riscv64 wheels

[luhenry]

• Package: cartopy
• Version: 0.25.0
• Source: https://github.com/SciTools/cartopy
• Docs: https://scitools.org.uk/cartopy/docs/latest/

Compiles cartopy.trace, a single Cython/C++ extension that combines shapely and pyproj (pure Python calls, no direct GEOS/PROJ linking) to project geometries. Upstream publishes no riscv64 wheel.

Mirrors the build_wheels job of upstream's release.yml.

Differs from upstream

• No GEOS/PROJ build step - the extension only calls shapely/pyproj at the Python level.
• Test command replaced with upstream's own no-network job (ci-testing.yml) - hermetic subset.

Testing

• Same marker filter as upstream's own no-network CI job (-m "not natural_earth and not network"), without --mpl image comparison - matches that job too, and avoids font-rendering-sensitive baseline diffs.
• Tests copied out of the checkout since the wheel excludes cartopy.tests.

License: OK.

Built locally (macOS, cp312) against pyproj 3.7.2 (matches the registry's riscv64 build): 661 passed, 11 skipped, 2 xfailed.