Patch Pantheon cache plugin to fix issue with cache time applied to both browser and CDN

See [the underlying issue and invite for a PR here](https://github.com/pantheon-systems/pantheon-mu-plugin/issues/127).

Start with the underlying issue and PR invitation linked in the issue body: https://github.com/pantheon-systems/pantheon-mu-plugin/issues/127. Use that discussion to identify the Pantheon cache plugin entry point and expected cache behavior. Done means the cache time is no longer applied identically to both browser and CDN caching, with the relevant plugin checks passing.

Written by the indexing model from the issue text.