Trigger rust CI with changes under external-crates/ (MystenLabs#10455)

## Description 

Otherwise we may miss issues with changes under the directory.

## Test Plan 

CI

---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

.github/actions/diffs/action.yml

@@ -22,6 +22,7 @@ runs:
       filters: |
         isRust:
           - 'crates/**'
+          - 'external-crates/**'
           - 'narwhal/**'
           - '.github/workflows/bench.yml'
           - '.github/workflows/codecov.yml'