submodules: bump external/oasis-core from c4a993d to a17cbda

Bumps [external/oasis-core](https://github.com/oasisprotocol/oasis-core) from `c4a993d` to `a17cbda`.
- [Release notes](https://github.com/oasisprotocol/oasis-core/releases)
- [Commits](oasisprotocol/oasis-core@c4a993d...a17cbda)

---
updated-dependencies:
- dependency-name: external/oasis-core
  dependency-version: a17cbda4ee5f4ee2c74c9798fb2a49c797e9932d
  dependency-type: direct:production
...

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

Author: dependabot[bot]