Run tests for cross-arch builds via qemu

[geofft]

Over in uv we use uraimo/run-on-arch-action to test our cross-compiled artifacts on other arches using Docker + qemu-user-static. We should do the same in python-build-standalone - we currently skip running the interpreter tests (validate-distribution --run) on cross-arches. See e.g. https://github.com/astral-sh/python-build-standalone/actions/runs/16208167880?pr=694 which ought to be failing on all arches.