Resolve "S3_CACHE_INSECURE must always be populated, or not present when false"
S3_CACHE_INSECURE && CACHE_SHARED are both false by default. However, the presence of either in the format of VAR= will result in them being treated as boolean flags, where their existence means true, unless set explicitly to false.
This change ensures that the variables are not exported to the ENV unless their values are truthy.
Closes #24 (closed)