s3: fix IAM Role for Service Account not working and other auth problems

This patch reverts this commit

1605f9e14d s3: Fix shared_credentials_file auth

It unfortunately had the side effect of making the s3 SDK ignore the
config in our custom chain and use the default provider. This means
that advanced auth was being ignored such as --s3-profile with
role_arn.

Fixes #5468
Fixes #5762

backend/s3/s3.go

@@ -1603,6 +1603,9 @@ func s3Connection(ctx context.Context, opt *Options, client *http.Client) (*s3.S
 		awsSessionOpts.SharedConfigState = session.SharedConfigEnable
 		// Set the name of the profile if supplied
 		awsSessionOpts.Profile = opt.Profile
+		// The session constructor (aws/session/mergeConfigSrcs) will only use the user's preferred credential source
+		// (from the shared config file) if the passed-in Options.Config.Credentials is nil.
+		awsSessionOpts.Config.Credentials = nil
 	}
 	ses, err := session.NewSessionWithOptions(awsSessionOpts)
 	if err != nil {