Inject nohup to make sure that bkill executes after ert dies

[xjules]

Issue After initiating terminate experiment a window can become unresponsive and thus user might ctrl+c / kill the main ert. nohup makes sure that the bkill will be executed.

Approach Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• [ ] PR title captures the intent of the changes, and is fitting for release notes.
• [ ] Added appropriate release note label
• [ ] Commit history is consistent and clean, in line with the contribution guidelines.
• [ ] Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• [ ] When there are user facing changes: Updated documentation
• [ ] New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• [ ] Large PR: Prepare changes in small commits for more convenient review
• [ ] Bug fix: Add regression test for the bug
• [ ] Bug fix: Create Backport PR to latest release