[TM-71] Fix CI triggering

Description

Problem: CI isn't triggered when something inside src/*/... is changed.

Solution: Update .gitlab-ci.yml

For more information see: https://docs.gitlab.com/ee/ci/yaml/#onlychangesexceptchanges

Related issue(s)

https://issues.serokell.io/issue/TM-71

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock
    • docs/

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.