infra: unset AWS_DEFAULT_REGION in release pipeline (#443)

Release pipeline is
[blocked](https://github.com/langchain-ai/langchain-aws/actions/runs/14117529781/job/39551249220)
(this is a guess at a solution).

Author: ccurme

.github/workflows/_release.yml

@@ -196,8 +196,11 @@ jobs:
           unset AWS_ACCESS_KEY_ID
           unset AWS_SECRET_ACCESS_KEY
           unset AWS_REGION
+          unset AWS_DEFAULT_REGION
           # Verify they are removed
           if [ -z "$AWS_ACCESS_KEY_ID" ]; then echo "AWS_ACCESS_KEY_ID is unset"; else echo "AWS_ACCESS_KEY_ID is still set"; fi
+          if [ -z "$AWS_REGION" ]; then echo "AWS_REGION is unset"; else echo "AWS_REGION is still set"; fi
+          if [ -z "$AWS_DEFAULT_REGION" ]; then echo "AWS_DEFAULT_REGION is unset"; else echo "AWS_DEFAULT_REGION is still set"; fi
           
           make tests
         working-directory: ${{ inputs.working-directory }}