How do I generate an index file the best way?

[pale shale]

I have a program that is kind of like type-racer but for code instead of text, and I plan to have a folder on github with various challenges that are just pairs of text files, where the user has to turn one file into another
I'm planning on just fetching these files from github, but rather than fetching all the challenges which would be slow and im pretty sure would quickly ratelimit me, I want to have a single index.json file that has all the data of all the challenges, then the user picks one/gets one at random and I just fetch the files for that challenge

but I'm not sure what the right way to generate this index.json is? the way I'm planning, this isn't something packaged with the binary, it'll be fetched from github.

[valid furnace]

well regenerate it on every commit probably ?

it should be able to be done automatically

[pale shale]

yeah that's the idea but is there a library or some idiom to do this correctly?
I thought I'd want build.rs but I don't believe I can use code in src/ in build.rs

[valid furnace]

don't think there is ? a standalone rust file should do the trick pretty easily.
what i would say matters is link it to commits, not build