_project.py: Allow junctions to use parent remote when `ignore-junction-remotes` set
Description
When you have a remote cache configured in project.conf and a junction element to a project with a remote cache configured, you can currently ignore the remote cache from the junction. Unfortunately, at the moment this also stops artifacts from the junction being cached in the remote configured in the project.
For some motivating examples of when this is a problem, consider a project A that junctions some project B, which has an incompatible cache configured. If you want to remove the warning that there is an incompatible remote with almost every bst invocation, you now lose the ability to cache the junctioned elements. If you rely on this for, say, a toolchain, then you basically lose a lot of cache. Alternatively, I may just want to use my own cache for cross-junction artifacts for connectivity reasons or trust.