[EXPERIMENT] Make CI use split repo

[cstamas]

And then reuse Central cache across jobs, while the rest (locally installed) are throwaway.

Changes:

• rework CI to use MAVEN_ARGS to make all Maven invocations use split repo, CI caches now only central
• EmbeddedExecutor is now equivalent to forked: it also observes MAVEN_ARGS env variable (forked was always, as it is mvn script invoked).
• Helper test made "resilient" to split repo: it now asserts "ends with" as split repo may add prefixes to local repository paths.
• Verifier methods made "repository aware" (null = installed, non-null = remote repositoryId)
• Tests doing artifact LRM ops now need to be aware of their origin

Result of these changes should be drastically lower pulls from Central, ideally zero in fact (only when a dependency changes).

[cstamas]

Giving up this work, we still end up w/ dirty central caches as:

• split repo is only about "repoId"
• but many ITs redefine "central" as local file repo -> those would end up polluting cache
• some ITs redefine "centra" with bringing up Jetty -> same thing as above, but goes over resolver