internal/kokoro: artifact extension now .txt, not .log
internal/kokoro: artifact extension now .txt, not .log

in order to get the gcs content-type to be text/plain,
we must change the file extension to .txt.
kokoro will guess the content-type based on file extension.

Change-Id: If3f9697537817ff0ad7f0c8bea77cfc0497fafe3
Reviewed-on: https://code-review.googlesource.com/16430
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: JBD <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go