[JENKINS-74005] Obtaining login button via href instead of text

[PereBueno]

https://issues.jenkins.io/browse/JENKINS-74005
Switched how Log in link was obtained, so now it looks for the href attribute, which will always be the same regardless the button text changes.

Testing done

Runned ATHs with no problem

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue