RHOAIENG-17257: chore(tests): externalize the default workbench-starting arguments

[jiridanek]

https://issues.redhat.com/browse/RHOAIENG-17257

Description

This is the last small improvement/refactoring, I promise.

How Has This Been Tested?

• https://github.com/jiridanek/notebooks/actions/runs/12907492615

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[jstourac]

/lgtm

[jiridanek]

/approve

[openshift-ci]

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[jiridanek]

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[openshift-ci]

New changes are detected. LGTM label has been removed.

[jiridanek]

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