Downgrade to Ubuntu 20.04 on sparc64

...because of glibc version mismatching between Debian and Ubuntu.

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

ci/docker/sparc64-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,5 @@
-FROM ubuntu:22.04
+# FIXME: Update to 22.04 once Debian image of sparc64 has a newer glibc.
+FROM ubuntu:20.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         curl ca-certificates \