This removes the need for supplying a locally built telega-server
binary, as the new upstream derivation includes this.
It also ensures that the frontend/backend are synchronised, which I
think has been causing some issues on vauxhall.
Change-Id: If504624e607a24fa12d68516cde65fef25ed2838
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1749
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>