Change account + token used for myriad gha in prod

[gmanchon]

👉 the new ACCOUNT + TOKEN should be myriad gha specific so that we know what is impacted and what to test when the token is updated or removed (similarly to a GCP service account linked to an app)

current token has org:admin + repo + workflow

once myriad works with the latest challengify version we can remove the taxi-fare challengify inplace GHA + remove the workflow access right

from taxi fare PR challengify inplace doc :

the myriad gha must have the right to push a worflow file
https://github.com/lewagon/data-solutions/blob/master/.github/workflows/myriad.yml

   --gh-nickname "gmanchon" \
            --gh-token "${{ secrets.GMANCHON_GH_TOKEN }}" \
👆 this "myriad prod do not delete" token of the "gmanchon" account uses "admin:org, repo, workflow"