CI: Use GitHub org variable for AWS region

[victorlin]

Organization variable was recently added in nextstrain/zika#54.

Checklist

• Checks pass
• Post-merge: Remove AWS_DEFAULT_REGION repo secret

[victorlin]

On one hand, this reduces redundancy in having multiple AWS_DEFAULT_REGIONs defined. On the other hand, this moves AWS_DEFAULT_REGION away from other related variables, some of which must be kept as secrets:

Thoughts? Based on code search, this applies to nextstrain.org and forecasts-ncov.

[joverlee521]

We've had to remove AWS_DEFAULT_REGION from secrets for any repo that uses the pathogen-repo-build workflow to prevent the AWS batch link from being borked (e.g. nextstrain/rsv#53 (comment)). I think this move will match expectations that only "real" secrets should be kept as secrets and other shared values can be the unmasked variables.

[victorlin]

Thanks for the added context! I agree better to properly scope usage of the secrets store. I'll follow through with the move here and in forecasts-ncov.