Flag is in rustup-init.sh: 72b9821072/rustup-init.sh (L51-L62)
72b9821072/rustup-init.sh (L51-L62)
In the use cases where we want this, it needs to be specified on the command-line anyway. In the use cases where we don't want--such as ADDing the source to the builder image for a cross-machine build--we don't want it at all. So don't declare it.
stable
This seems to work fine locally.