Add cache-control header to objects in GCS

AnthonySandovalrequested to merge
fix-cache-about-src into master

Partially addresses gitlab-com/gl-infra/infrastructure#7902, at least for new and modified files.

We're currently not setting any Cache-Control headers, which are allowed. See https://cloud.google.com/storage/docs/gsutil/addlhelp/WorkingWithObjectMetadata.

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