submodules: bump external/cli from 6b400a2 to a63db76

Bumps [external/cli](https://github.com/oasisprotocol/cli) from `6b400a2` to `a63db76`.
- [Release notes](https://github.com/oasisprotocol/cli/releases)
- [Commits](oasisprotocol/cli@6b400a2...a63db76)

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

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