This changes the structure of the output, too, where all AoC solutions
now end up in a big folder with `bin/day$n` executables.
Change-Id: I77928f4129489d06779b50059835925652688c9c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2231
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI