CI: Speed-up rtp.io pipeline, handle job cancellations and notifications correctly, use latest debian

[sobomax]

Summary

Speed-up rtp.io pipeline, handle job cancellations and notifications correctly

Details

1. The current rtp.io/voiptests pipeline builds python wheels needed each time, which takes significant amount of time
2. debian-12 is getting old
3. The notification hook runs always even when the run has been cancelled or runs in a custom fork

Solution

1. Use pip wheels cached by the sippy pipeline
2. Switch to using debian-13.
3. Don't run notification if the job is cancelled, skip notification entirely if a custom fork or PR.