submodules: bump external/oasis-sdk from bc0a40b to e9991c7

Bumps [external/oasis-sdk](https://github.com/oasisprotocol/oasis-sdk) from `bc0a40b` to `e9991c7`.
- [Commits](oasisprotocol/oasis-sdk@bc0a40b...e9991c7)

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

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