CI builds (master and PR) now provide a p2 repo for testing output

[mickaelistria]

We've recently configured JDT Core so that all the builds made via Jenkins do generate a p2 repo containing the built artifacts. Those artifacts are meant for local integration testing: you may reference the generated p2 repo directly to try it in your build or product, by installing the freshly built artifact or referencing them in a .target file.
Note that those artifacts are meant only to facilitate testing of PRs or experimental branches that are under active development, they are not supported, probably not signed and haven't received the extra quality checks that are applied to the official SDK builds; and there is no plan to change that at the moment. Because the CI infra is not a file server and won't scale with too many requests, these p2 repo also must not be referenced in production nor in automated frequent builds, only sporadic and local consumption is recommended.

From a given Jenkins job, the p2 repo is $[jobURL}/artifact/repository/target/repository/, for example https://ci.eclipse.org/jdt/job/eclipse.jdt.core-Github/job/master/lastSuccessfulBuild/artifact/repository/target/repository/ for last build on master, or https://ci.eclipse.org/jdt/job/eclipse.jdt.core-Github/job/master/104/artifact/repository/target/repository/ for a particular build of master, or https://ci.eclipse.org/jdt/job/eclipse.jdt.core-Github/job/PR-2124/5/artifact/repository/target/repository/ to test the output of a particular pull request.