Use JENKINS_UC_DOWNLOAD_URL if provided

[nhojpatrick]

Fixes https://github.com/jenkinsci/plugin-installation-manager-tool/issues/415

The current code ignores JENKINS_UC_DOWNLOAD_URL if specified, as logic above will always result in something else being checked. Other if/else statements might also need to be changed to fix other issues with that logic.

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[timja]

can you take a look at the tests?

[nhojpatrick]

can you take a look at the tests?

Happy too, I've no time until weekend probably but will be looking at them then. Wanted to get it raised and review then fix tests.

[nhojpatrick]

Fixing the same issue as highlighted here https://github.com/jenkinsci/docker/pull/1299

[nhojpatrick]

I've found time. To make testing easier I've added JUnit v5 and assertAll to all errors show. I've updated so if you define JENKINS_UC_DOWNLOAD_URL it's used for everything, so a true global override.

[nhojpatrick]

Converting to draft and rebase from PR https://github.com/jenkinsci/plugin-installation-manager-tool/pull/417 so this PR easier to understand what has changed when using assertAll to improve dev/test cycles.