OTOH the save actions plugin provides autosave all on a timer, that may meet your requirements, or since there is a signal on build start (which actually notes a possible use is for a plugin to implement "save all") somebody could provide a PR to add a "save all on build" option to the save actions plugin.
That sounds like a great idea. I already had the _Save actions_ plugin installed, but had disabled it - if I recall correctly, because each 'save' also activates the 'remove trailing spaces' (if enabled). That means while I'm typing, trailing spaces get removed unexpectedly on each auto-save - but that's another issue...
Cheers!