Add sign to <activeProfiles>

[basil]

Fixes a regression exposed by jenkinsci/pom#281 and jenkinsci/jenkins#6914. In order for release artifacts to be signed by maven-gpg-plugin and maven-jarsigner-plugin during the release:prepare goal, we rely on https://github.com/jenkinsci/jenkins/blob/d25704cfbc929bca3bd5e5675e1fe1aa1a58928c/pom.xml#L521-L540 and https://github.com/jenkinsci/jenkins/blob/d25704cfbc929bca3bd5e5675e1fe1aa1a58928c/war/pom.xml#L623-L648, activated by the release and sign profiles respectively. To activate these profiles during the release:prepare goal we currently rely on https://github.com/jenkinsci/jenkins/blob/d25704cfbc929bca3bd5e5675e1fe1aa1a58928c/pom.xml#L428-L429, which happens to expose a serious bug in recent versions of Maven Release Plugin as explained in jenkins-infra/helpdesk#3143 (comment). Fortunately for us, we can work around the problem by omitting any custom arguments from our Maven Release Plugin configuration as in #291 and jenkinsci/jenkins#7138, instead enabling these profiles via an <activeProfiles> setting in settings-release.xml as in this PR. Note that we cannot enable these profiles using releaseProfiles because that setting only applies to the release:stage and release:perform goals, not the release:prepare goal.

[NotMyFault]

Thanks for the quick investigation!