Add Raspberry Pi build back to the nightly pipeline
We removed RPi builds from nightly pipeline in !3199 (merged) because they were built on scaleway and took hours to complete. However, we have since moved from Scaleway to Graviton, and the build times are now comparable to that of other OSs.
We should bring back RPi builds to nightly so that we catch any issues early. Else, unless omnibus-gitlab
gets an update to master
branch, build failures in RPi due to changes in other projects will go unnoticed.