[Platform CI] Speed up Maven builds with an internal Nexus proxy

Currently, we are fetching the Maven artifacts directly from packages.nuxeo.com, see https://github.com/nuxeo/platform-ci/blob/v0.0.49/charts/jenkins-extra/values.yaml.gotmpl#L25.

We will probably save some download time by going through a Maven proxy repository in the internal Nexus, hosted in the same Kubernetes cluster as the Jenkins agent running the mvn command.

This way, once the artifacts are cached from packages.nuxeo.com to this internal Nexus (https://nexus.platform.dev.nuxeo.com/), the downloads will be done from GCP to GCP instead of GCP to AWS (where packages.nuxeo.com lives in EKS).