Bug 1618755 - Add unique key attributes for TierIndicator buttons (#6072)

This prevents React from complaining about the missing key attributes.

ui/job-view/headerbars/TierIndicator.jsx

@@ -22,6 +22,7 @@ export default function TierIndicator(props) {
             }
             disabled={isOnlyTier}
             outline={!shownTiers.includes(tier)}
+            key={tier}
           >
             {tier}
           </Button>