Port builds to pathogen-repo-build

[joverlee521]

Originally part of checklist in nextstrain/.github#46.

This would require porting all of the rebuild-*.yaml GH Action workflows to use the shared pathogen-repo-build workflow.

[tsibley]

In doing this, the workflows should switch away from the long-lived IAM user to short-lived role sessions to access AWS.

• Remove AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY from repo secrets.
• Remove --env AWS_ACCESS_KEY_ID and --env AWS_SECRET_ACCESS_KEY lines from nextstrain build invocation (if any)
• Delete nextstrain-ncov-runner IAM user and associated policies