prepare-release now runs flux push artifact to publish the rc's packages
artifact, and it installed whatever flux was latest, while build-amd64
and the candidate checks pin one version. It installs that version now.
The arm64 leg and the nightly never run flux, so they no longer install
it. The rc-e2e comment gave a stable tag as the reason for its
!cancelled(), but the job only runs for an rc; it now says it follows
the rule every job below build-arm64 has.
Assisted-by: LLM
Signed-off-by: Aleksei Sviridkin <f@lex.la>