Update docs on how to use env variables for S3 cache access

Stan Hurequested to merge
sh-update-s3-aws-env-docs into main

What does this MR do?

GitLab Runner v17.5.0 switched the S3 client to use the AWS SDK v2 (!4987 (merged)). As a result, this takes advantage of the default credential chain that enables S3 credentials to be supplied by environment variables, local files, or IAM profiles. Update the documentation to reflect this reality.

Relates to #26871 (closed)

Edited by Stan Hu

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