`--attach` generates a toml with an odd structure

[fliepeltje]

Describe the bug

When I use the --attach flag, this is the fly.toml that I get in my fly/apps/<app-name>/fly.toml

[experimental]
  [experimental.attached]
    [experimental.attached.secrets]
      [experimental.attached.secrets.export]
        SANDWICH_CHAT_URL = 'http://${FLYCAST_URL}/sandwich-chat'

The relevant section of the original fly.toml for the attached app looks like this:

[experimental.attached.secrets.export]
  SANDWICH_CHAT_URL = "http://${FLYCAST_URL}/sandwich-chat"

Functionally this of course works, but it looks a bit weird.