Alert on failure to update flake inputs

[stackptr]

This failed for 3 weeks as my attention has been elsewhere (job hunting is hard): #229

There is facility to setup notifications for workflow runs but after trawling through GitHub docs pages it's unclear how granular this can be configured. A simpler approach could be to send a push to my devices via Pushover, though both actions seem unmaintained. I already send notifications via curl for torrent completions but doing it in an action would require storing the required tokens in both GitHub and the existing .age file.