CI: Use Ubuntu mirrors instead of azure (#18057)

Use the official Ubuntu apt mirrors instead of
azure.archive.ubuntu.com, since that mirror can be slow:

https://github.com/actions/runner-images/issues/7048

This can help speed up the 'Setup QEMU' stage.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #18057