Tag rustup artifacts with surrogate key

[jdno]

We use surrogate keys^1 on Fastly to invalidate the cache for specific objects, e.g. when new versions of Rust or rustup are released. Surrogate keys are transparently set in the Fastly configuration based on the request path.

A new VCL snippet has been added that tags every rustup artifact with the rustup surrogate key so that we can purge them from the cache when a new rustup version is released.

This is part of #415.