Draft: Add env for Run 3 rJpsi in RD

Jamie Goodingrequested to merge
jagoodin/rd-rjpsi-run3 into master

lb-conda environment for RD Run 3 measurement of r_{J/\psi} (https://gitlab.cern.ch/LHCb-RD/ewp-rjpsi-2024)

Cc @cmarinbe @rquaglia

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