CI caches are invalid for pull requests

[ferki]

Describe the bug

558aa9a introduced a CACHE_VERSION variable to the CI cache key, so the cache could be invalidated by changing the value of that variable (=without committing to the codebase). This variable is defined as a repository secret, which is not available for pull requests from forks.

This causes using a brand new cache for PR from forks, which also means e.g. empty list of known perlcritic violations, leading to accepting the new state whatever it is (via Test::Perl::Critic::Progressive thinking it is its first run).

How to reproduce it

Steps to reproduce the behavior:

1. Fork the repo
2. Send a pull request
3. Check the cache key value in the build log

Shortest code example that demonstrates the bug:

Expected behavior

Pull requests from forks should use the same caches as internal pull requests (and should be tested the same way too).

Circumstances

Debug log