Add caching to CI operations

[jonastheis]

Description of change

This PR improves runtimes of our CI workflows for unit tests and integration tests by leveraging Github Actions Cache.

• unit tests: cache Go modules and build cache
• integration tests
  • cache Go modules and build cache for tester container
  • reuse Go buster image that is also used for building GoShimmer
  • remove "pulling additional images" step where not necessary

Note: Loading the same cache on multiple jobs in the same workflow causes some issues because the first runner will create the cache and following runners will get a cache hit. However, as long as the first runner did not yet finish its execution the cache will be empty. That's why I opted for an approach to always fall back to another cache with restore-keys. This might lead to very frequent cache eviction from Github because of the 5GB max cache size. I guess we have to see how it goes in general.

Note 2: experiments showed that caching the Docker image layers with Buildkit is not really worth it as long as exporting the internal Buildkit cache can't be exported. Importing/exporting the cache + rebuilding the binary is simply more expensive than just rebuilding the full image from scratch (see here for more information).

Runtime integration tests with caching of integration tests (tester Go dependencies and build cache) (left) vs without caching (right)

Runtime unit tests with caching of Go modules and build cache (left) vs without caching (right)

Fixes #1689

Type of change

• Enhancement (a non-breaking change which adds functionality)

[johanneskeim]

Looks good!

[georgysavva]

BTW, are we done with the elimination of large time.Sleep() in integration tests? I guess it should give a much greater performance increase.