Fix alpine permission and refactor test

What does this MR do?

Fix alpine's issue #336314 (closed) - test browser_ui/6_release/deploy_key/clone_using_deploy_key_spec.rb. The apk tool sometimes fells due to permission denied, but we don't necessary has to install agent all the time, so we should check if agent is installed first and skip the installation if it's there.

I also did some refactor around this test as well to keep it align with current style guide. But more importantly avoiding using tags = [qa, docker] due to #331753 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #336314 (closed)