Revisit updating AWS CLI to v2

[joverlee521]

Revisit #214 once we have the stomach for dealing with fallout of the new requirement for AWS_DEFAULT_REGION in AWS CLI v2.

[jameshadfield]

Totally understand this is a pain to add, but I've run into bugs and suggested we do this a few times, and I continue to think it'd be wise to make it an explicit variable (although not a secret!) throughout our workflows, even in the absence of this issue. Given the requirements of AWS CLI v2 it seems like we should start adding these to workflows / envdirs / etc whenever we notice it's not set.

[victorlin]

AWS CLI v2 automatically handles AWS_ENDPOINT_URL, so af704b6 can be reverted with the update.

[joverlee521]

Revisiting this because of nextstrain/infra#72 (comment).

All of the automated workflows should be running with the shared pathogen-repo-build workflow, which setsAWS_DEFAULT_REGION.