ci: add pull-through caches for dependencies to mitigate transient errors

Created by: jhchabran

Context: https://sourcegraph.slack.com/archives/C07KZF47K/p1634591872474600

A common source of CI flakes are failed downloads of mandatory dependencies, which have various causes: transient network errors, rate-limiting from Github, etc ... Those can be addressed individually, but on the long run, a more general solution would definitely be best.

Possible solutions:

• introduce HTTP caching through a proxy, to avoid cluttering the build scripts with caching logic
• cache various build artefacts (node modules, binaries, etc..)
• ..

⚠️ There are many ways we can address this, let's make sure the problem is properly scoped before looking for a solution.