[CO-400] Fix exit code acquiring in nix integration tests

[KtorZ]

Description

Since a1e7e80 introduced a pipe after stack test, we were
wrongly getting the status from the tee command instead.

Linked issue

[CO-400]

Type of change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🛠 New feature (non-breaking change which adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
• 🔨 New or improved tests for existing code
• ⛑ git-flow chore (backport, hotfix, etc)

Developer checklist

• I have read the style guide document, and my code follows the code style of this project.
• [ ] If my code deals with exceptions, it follows the guidelines.
• I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on cardanodocs.com, amending the inline Haddock comments, any relevant README file or one of the document listed in the docs directory.
• [ ] CHANGELOG entry has been added and is linked to the correct PR on GitHub.

Testing checklist

• [ ] I have added tests to cover my changes.
• All new and existing tests passed.

QA Steps

Screenshots (if available)

[paweljakubas]

should be rebased to accomodate #3678
In https://hydra.iohk.io/build/260699/log we see that as some tests fail the integration test is red, rather than green as previously

[KtorZ]

No need to rebase here. We can just merge it into develop 👍, CI just picked an old develop.