Fix automation hash

[joverlee521]

Description of proposed changes

Adds AWS_DEFAULT_REGION envvar to the check-new-data jobs to ensure that the aws s3api head-object calls work properly. I always forget this needs to be set in a brand new environment 🤦‍♀️

Outside of this PR, I also added AWS_DEFAULT_REGION to the shared Nextstrain GitHub organization variables.

Related issue(s)

• Follow up on #52
• Resolves #53

Checklist

• [x] Checks pass
• [x] Test run used the correct hashes from S3 files

[joverlee521]

Merging pre-review to fix our automated run. Welcome any post-merge feedback!