pr_check: use insights-stage as ref-env

[jrusz]

This will deploy the apps using whatever is in stage which is where PRs are deployed after being merged.

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[app-sre-bot]

Can one of the admins verify this patch?

[ezr-ondrej]

/ok-to-test

[ezr-ondrej]

/retest

[ezr-ondrej]

Sure, I'm ok with this.
I'll wait for @adarshdubey-star ack here, but IMO testing against stage makes sense.