var/build_id should be stable accross build machines
var/build_id value, which is included in filenames, is a hash of all the inputs used to create a build.
In theory this hash should be the same on different build machines. However it seems it is not always the case, at least for the android build, and we should find out why.
This command can be used to see the filename of Tor Browser alpha for android-armv7:
./rbm/rbm showconf tor-browser filename --target alpha --target torbrowser-android-armv7