replace rhel8 builder pullspec after great purge

[grokspawn]

CI failures around branch cut resulted in a lot of images being purged from test registries. This switches the rhel8 builder to use the canonical location (which was purged) going forward.

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[grokspawn]

Hmm.... looks like this may not be necessary anymore, as the builds of operator-framework-tools seems to be passing now (implying that the image was re-loaded into the registry).