Calculate a default key about cache

[hugovk]

Here's what Travis does:

When you have multiple jobs in a build matrix, some characteristics of each job are used to identify the cache each of the jobs should use.

These factors are:

1. OS name (currently, linux, osx, or windows)
2. OS distribution (for Linux, xenial, trusty, or precise)
3. macOS image name (e.g., xcode7.2)
4. Names and values of visible environment variables set in .travis.yml or Settings panel
5. Language runtime version (for the language specified in the language key) if applicable
6. For Bundler-aware jobs, the name of the Gemfile used

If these characteristics are shared by more than one job in a build matrix, they will share the same URL on the network. This could corrupt the cache, or the cache may contain files that are not usable in all jobs using it. In this case, we advise you to add a public environment variable name to each job to create a unique cache entry:

CACHE_NAME=JOB1

https://docs.travis-ci.com/user/caching/#caches-and-build-matrices

So only in very rare situations would you need to define a custom key/name for the cache, and I've never needed to do so.

An example cache zip file from a build is master/cache-linux-xenial-e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855--python-3.7.tgz.

Thanks! (And thanks for adding caching, it's already speeding up builds considerably! For example, one 98s step -> 27s including cache steps.)

[chrispat]

Thank you for the feedback,

GitHub Actions and Travis CI have pretty fundamentally different models. Travis is based around a relatively fixed model of languages and shell scripts and actions is much more generic and open.

Take matrix for example. In GitHub Actions we have no idea what those matrix keys are going to be as that is completely up to the user and the individual action can't use information from any context other than what is supplied by the user.