chore(deps): bump actions/cache dependency version to v4

[LNSD]

Context

With Node 16 reaching End-of-life, Github actions deprecated it as runner of the GitHub actions on Sept 2023 (see: GitHub Actions: Transitioning from Node 16 to Node 20
).

Proposed fix

To fix the deprecation warning, this PR updates the actions/checkout action to the new major version, v4, using the new runtime recommended version: Node 20.

[LNSD]

Additionally, the actions/cache action in the latest released version supports the save-always flag, which would resolve #14

_{Source: https://github.com/actions/cache/tree/main#v4}

[alice-i-cecile]

Thanks! I'd welcome a PR to add the save-always flag as well.