Use a more specific informational name for customUrl

[smlambert]

Relating to this work/discussion, #6694 (comment), we should improve the variable name 'customUrl' so that we better understand the Jenkinsfile code upon reading it. We use a lot of URLs for a lot of different reasons, so we need to have better naming to make the code clearer and more understandable.

[smlambert]

Fixed via #6909 and adoptium/TKG#812, thank you @Amrutha-Kanhirathingal