External collaborator PRs fail CI build

[vcarl]

See this run for instance:

#18 ERROR: denied: installation not allowed to Write organization package
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 > pushing ghcr.io/reactiflux/reactibot:pr-215 with docker:
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error: denied: installation not allowed to Write organization package
Error: buildx failed with: error: denied: installation not allowed to Write organization package

[vcarl]

From dupe:

#19 ERROR: denied: installation not allowed to Write organization package

This appears consistently for all PR builds from forks because it's trying to publish a build artifact to the reactiflux scope without having permissions. Realistically we can probably stop publishing PR builds, I set it up that way so we'd have the option of running the exact artifact locally but in practice it's not something we've done (ever?)