ci: pin executor image used to build it

Created by: jhchabran

See https://bugs.launchpad.net/ubuntu/+source/linux-aws/+bug/1977919 (kudos to @rafax for finding the bug report 🙏 )

Basically, today's build causes a kernel panic when using Docker.

Test plan

Ran the build script locally, it passed with the image pinned to yesterday's version.