fix: build: an epoch is near an upgrade iff the upgrade is enabled (#11401)

This isn't a huge deal, but we'd otherwise consider the first finality
epochs (or so) to be "near" any disabled upgrade.

build/isnearupgrade.go

@@ -5,5 +5,8 @@ import (
 )
 
 func IsNearUpgrade(epoch, upgradeEpoch abi.ChainEpoch) bool {
+	if upgradeEpoch < 0 {
+		return false
+	}
 	return epoch > upgradeEpoch-Finality && epoch < upgradeEpoch+Finality
 }