CloudFront invalidations for docs-rs staging

[rylev]

The docs-rs staging environment does not support for CloudFront invalidations which happens through the CLOUDFRONT_DISTRIBUTION_ID_WEB and CLOUDFRONT_DISTRIBUTION_ID_STATIC environment variables. These needed to be added to the web and background builder.

[syphar]

actually, if I'm not missing something,

only the builder & the registry watcher need these variables for invalidations.

[Mark-Simulacrum]

FWIW long-term I expect we'll want to make sure that environments executing arbitrary code (i.e. the builder) have fairly locked down access, ideally not direct access to database and APIs like cloudfront etc.

Definitely not a blocker to doing this, just wanted to note somewhere.