On unstable there's the "-Z no-index-update" flag which "ensures that Cargo does not attempt to update the registry index. (...) to avoid the network latency for updating the index each time"
Following stable options are often more then enough:
`--locked` might not prevent index updates, but prevent any changes in used code, i.e. only what is in the lockfile will be used
`--frozen` like locked but also requires the (download?) cache to be up to date
`--offline` prevent index updates and downloads
So if you have everything in the download cache `--offline` is like `-Z no-index-update` and `--locked` is most times good enough because since partial indices where added the index updates don't consume much time at all.
https://doc.rust-lang.org/nightly/cargo/reference/unstable.h...