submodules: bump external/oasis-sdk from 6590ae6 to d1b0bb6

Bumps [external/oasis-sdk](https://github.com/oasisprotocol/oasis-sdk) from `6590ae6` to `d1b0bb6`.
- [Commits](oasisprotocol/oasis-sdk@6590ae6...d1b0bb6)

---
updated-dependencies:
- dependency-name: external/oasis-sdk
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>