Previously, `weed mini` logic duplicated the credential loading process
by creating a temporary IAM config file from environment variables.
`auth_credentials.go` also had fallback logic to load these variables.
This change:
1. Updates `auth_credentials.go` to *always* check for and merge
AWS environment variable credentials (`AWS_ACCESS_KEY_ID`, etc.)
into the identity list. This ensures they are available regardless
of whether other configurations (static file or filer) are loaded.
2. Removes the redundant file creation logic from `weed/command/mini.go`.
3. Updates `weed mini` user messages to accurately reflect that
credentials are loaded from environment variables in-memory.
This results in a cleaner implementation where `weed/s3api` manages
all credential loading logic, and `weed mini` simply relies on it.
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