Allow AWS bucket name to be provided as an environment variable

For certain migration paths, a blob storage provider must be configured in the CLI for storing the migration archive and transferring it to GitHub.

When using AWS, all of the required blob storage connection settings can be provided using environment variables _or_ CLI arguments, _except_ the S3 bucket name which can only be provided using `--aws-bucket-name`.

Conversations with customers suggest that people are finding this confusing - "why can I specify everything AWS-related as an environment variable except the bucket name?". We should add the ability for this also be be specified in the environment.

## Todo

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## Dependencies

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