S3 publisher no longer resolves credentials from ~/.aws/config

Changed in 26.13.0 with the the move to a JS based S3 publisher
he previous AWS go SDK used to also check `~/.aws/config` for credentials, if the ` AWS_SDK_LOAD_CONFIG=1` is set.
this breaks existing pathway if you were using IAM Identity Center / SSO sessions for authenticating into AWS.

docs still point to this as being a supported feature: https://www.electron.build/docs/publish/#s3

previous related ticket where this auth pathway was added to the docs: https://github.com/electron-userland/electron-builder/pull/9018

not sure if the regression was intentional or if the docs need to be updated to reflect this.

Start by comparing the current JS-based S3 publisher's credential loading with the previous Go SDK behavior, using AWS_SDK_LOAD_CONFIG=1 and an IAM Identity Center/SSO session. Inspect the S3 publishing documentation and linked PR 9018; done means either restoring supported config resolution with appropriate coverage or updating the docs to match confirmed behavior.

Written by the indexing model from the issue text.