* build(just): factor out flags code

This will enable making use of JUSTFLAGS in the main Makefile for
migrated targets.

* fix(just): parallel support

For some unclear reason runs in CI environment are invalid (they
generate some "empty" calls to the private target).
Sidestep the issue for now.

* build(just): add more helpers

- VERSION_META variable
- go_generate target

* build(op-proposer): migrate build to just

* build(op-node): migrate build to just

* fix(build): rewire just-migrated targets

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Co-authored-by: Matthew Slipper <me@matthewslipper.com>