Fix bugs about the exit code (#262)

-   **fix: Make `fail: false` effective even when `failIfEmpty: true`**

This commit also makes sure `outputs.exit_code` is “The exit code returned from Lychee”. `failIfEmpty` no longer changes it to `1`.

Relevant docs:
- [Setting exit codes for actions - GitHub Docs](https://docs.github.com/en/actions/sharing-automations/creating-actions/setting-exit-codes-for-actions)
- [exit - POSIX Programmer's Manual](https://manned.org/exit.1posix)

Relates to #86, #128, #145, #245, and #251.

-   **fix: Update `env.exit_code` to `outputs.exit_code`**

The previous expression always gives `false`.
Both `env.exit_code` and `env.lychee_exit_code` are `null`, probably since the docker→composite refactor #128. When GitHub evaluates the expression, it finds the types do not match, and coerces them to number, namely, `null` → `0`.

See [Evaluate expressions in workflows and actions - GitHub Docs](https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/evaluate-expressions-in-workflows-and-actions#operators).

Relates to #253.



Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>