Allow running bash scripts

[ppcano]

Currently, the k6 action uses the k6 Docker image, which has k6 as its ENTRYPOINT. As a result, users cannot execute shell/bash scripts when using this action. This limitation restricts the usability of this action.

A more versatile alternative is that the action installs k6 and any dependencies in the running image. For reference, see:

• https://k6.io/blog/load-testing-using-github-actions/#using-a-different-runner
• https://github.com/grafana/k6-github-action
• https://github.com/grafana/quickpizza/blob/main/.github/workflows/k6-tests.yaml