There is the test environment for iOS builds linked previously. Once we are ready to put things into production the sources will be made available as a PR and merged in accordance with our open source development process, at which point anyone on macOS could build and run their own copy of App Inventor with the iOS build capability.
This is something that has crossed my mind on more than one occasion.
Based on the stats we have been collecting on the test server, we estimate that we should be able to service roughly 5 builds per minute per core, and we have 38 cores available of Apple hardware. For comparison, we run 168 cores total (21 machines x 8 cores) to support Android builds. Given our current user numbers, I think this is possibly enough to support the release of the iOS build functionality. There is the separate issue of squashing the many bugs identified by others before we release.
The major challenge with the "local first" build approach in my opinion will be ensuring folks keep up to date with App Inventor. Whenever we release a new version that changes the components (and triply so if we're doing independent Android and iOS releases or do a fix for bugs affecting compiled apps only), we redeploy the buildserver infrastructure with the latest code. This ensures that builds are always using the right backend and reduces the debugging load. If folks run their own server we have to ensure everyone updates otherwise we might end up trying to debugging an issue that's already been fixed.