Prefer per plugin url over JENKINS_UC_DOWNLOAD_URL

[timja]

See #416 (comment)

Testing done

Ensured that:

export JENKINS_UC_DOWNLOAD_URL=http://localhost:8080
java -jar plugin-management-cli/target/jenkins-plugin-manager-2.13.2-SNAPSHOT.jar --plugins mailer:1.34:https://get.jenkins.io/plugins/mailer/1.34/mailer.hpi -d /tmp/plugins --jenkins-version 2.440 --verbose

downloads from get.jenkins.io

and ensured:

export JENKINS_UC_DOWNLOAD_URL=http://localhost:8080
java -jar plugin-management-cli/target/jenkins-plugin-manager-2.13.2-SNAPSHOT.jar --plugins mailer:1.34 -d /tmp/plugins --jenkins-version 2.440 --verbose

downloads from localhost:8080

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