ci: stop using unsupported Ubuntu image in "linux32" job

The "linux32" job is currently using the "i386/ubuntu" image. This image has not received any updates for more than two years by now as Ubuntu dropped support for the 32 bit architecture completely.

Convert the job to use Debian instead. This distribution still supports 32 bit architectures and is as close as it gets to Ubuntu.

--- b4-submit-tracking ---

This section is used internally by b4 prep for tracking purposes.

{ "series": { "revision": 1, "change-id": "20250922-pks-ci-update-linux32-job-855730628301", "prefixes": [] } }