Fix: untaint s3 cache urls since they are output to the script

It seems config values that are passed from the worker (when the script is generated inline) are tainted. This makes the enterprise worker fail to compile the script.

I believe the only options that eventually end up in a string that is output to the script are used to generate the S3 cache urls. This change fixed this problem.

I don't quite understand why the travis-build service doesn't also blow up on this, but I think this change is safe to be merged anway.