artifacts: Destination for "local" artifact can be anything

Limiting the `destination` of a local artifact to a relative path
doesn't afford any benefit and is easily circumvented (using a two step
copy via a temporary variant for instance).

Remove the `relativelocal` validation from `destination` and allow users
to copy directly to any path within the build container.
3 jobs for !29 with review/allow-local-copy-destination-to-anywhere in 4 minutes and 11 seconds (queued for 27 seconds)
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