m/installer: fix minimum disk calculation for A/B partitions

Since the introduction of A/B updates two system partitions are created. 
Reflect that in the minimum root disk size calculation.

Change-Id: I3ad875bbae07e539b68f7a32409e5fdd4105a998
Reviewed-on: https://review.monogon.dev/c/monogon/+/2868
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Vouch-Run-CI: Leopold Schabel <leo@monogon.tech>
diff --git a/metropolis/installer/main.go b/metropolis/installer/main.go
index 8aa8836..6313548 100644
--- a/metropolis/installer/main.go
+++ b/metropolis/installer/main.go
@@ -238,7 +238,7 @@
 	}
 	// Calculate the minimum target size based on the installation parameters.
 	minSize := uint64((installParams.PartitionSize.ESP +
-		installParams.PartitionSize.System +
+		installParams.PartitionSize.System*2 +
 		installParams.PartitionSize.Data + 1) * mib)
 
 	// Look for suitable block devices, given the minimum size.