fix: Pin @turbo/gen version to match the running turbo binary (#11822)
## Summary
- `turbo gen` was shelling out to `npx @turbo/gen@latest`, meaning the
generator version could drift from the turbo binary the user is actually
running
- Changed the default `--tag` from `"latest"` to the compile-time
version from `version.txt` (via `get_version()`), so `@turbo/gen` always
matches the turbo binary
- Explicit `--tag` flag still works as an override