Use Go Module Cache For Tests in CI

[pavelbrm]

Summary

This PR slightly improves the tests run time in CI by utilising GitHub Actions cache for Go modules.

With this PR, the overall run time for a full CI starting from the second push to a branch with a PR is around 12 minutes (I saw a couple of runs going as low as 11:47).

Our current development and CI tooling is unnecessarily complicated – Docker is abused without strict need for it in CI. That means we can't simply benefit, for example, from caching.

However, I've managed to find a way to use cache. It does not speed things up too much, because the majority of time is spent in tests, but we at least no longer have to re-download dependencies on subsequent pushes to a branch with a PR.

One place that is still not using cached modules is the build step in the Dockerfile. Ideally, that build step has to happen outside. Thankfully, that should be achievable, and I will look at that some time in the future. That might help to cut a few more minutes.

Type of Change

• Product feature
• Bug fix
• Performance improvement
• Refactor
• Other

Tested Environments

• Development
• Staging
• Production

Before submitting this PR:

• Does your code build cleanly without any errors or warnings?
• Have you used auto closing keywords?
• Have you added tests for new functionality?
• Have validated query efficiency for new database queries?
• Have documented new functionality in README or in comments?
• Have you squashed all intermediate commits?
• Is there a clear title that explains what the PR does?
• Have you used intuitive function, variable and other naming?
• Have you requested security / privacy review if needed
• Have you performed a self review of this PR?

Manual Test Plan:

[pavelbrm]

According to Dockerfile recommendations, the fewer the layers, the better.