ubuntu-test: Explicitly set the platform that we're going to emulate.

This is to avoid 'docker: no matching manifest for linux/amd64 in the manifest list entries.'

Unclear why I don't have to do this for the manylinux images.

Author: jamadden

ubuntu-test

@@ -47,4 +47,4 @@ if [ -d /greenlet ]; then
 fi
 
 # Mount the current directory as /greenlet
-docker run --rm -v "$(pwd):/greenlet"  ${DOCKER_IMAGE:-riscv64/ubuntu:latest} /greenlet/$(basename $0)
+docker run --rm --platform linux/riscv64 -v "$(pwd):/greenlet"  ${DOCKER_IMAGE:-riscv64/ubuntu:latest} /greenlet/$(basename $0)