zed-local: add --stable flag to zed-local (#10165)

`--stable` makes all clients except for the first one use a stable
version of Zed (hardcoded to `/Applications/Zed/Contents/MacOS/zed` for
now).

That should make testing cross-channel collab changes a bit easier. /cc
@maxbrunsfeld @ConradIrwin

Release Notes:

- N/A