Fix bug in `update-sdk-next.yml` (#2746)

## Motivation and Context
The workflow is failing to update the aws-sdk-rust/next branch due to
the `tests/` folder in the repository root now.

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.github/workflows/update-sdk-next.yml

@@ -47,7 +47,7 @@ jobs:
         git checkout origin/main -b next
 
         # Delete the old SDK
-        rm -rf sdk examples
+        rm -rf sdk examples tests
         rm -f versions.toml Cargo.toml index.md
 
         # Copy in the new SDK