logictest: disable metamorphic testing for mixed version test

We've seen that these tests can get quite slow sometimes. One theory is
that this is caused by metamorphic test settings, so this PR disables
them so we can determine if it helps with test stability.

Release note: None

pkg/sql/logictest/logic.go

@@ -1303,11 +1303,14 @@ func (t *logicTest) newTestServerCluster(bootstrapBinaryPath, upgradeBinaryPath
 		testserver.CockroachLogsDirOpt(logsDir),
 	}
 	if strings.Contains(upgradeBinaryPath, "cockroach-short") {
-		// If we're using a cockroach-short binary, that means it was
-		// locally built, so we need to opt-out of version offsetting to
-		// better simulate a real upgrade path.
 		opts = append(opts, testserver.EnvVarOpt([]string{
+			// If we're using a cockroach-short binary, that means it was
+			// locally built, so we need to opt-out of version offsetting to
+			// better simulate a real upgrade path.
 			"COCKROACH_TESTING_FORCE_RELEASE_BRANCH=true",
+			// The build is made during testing, so it has metamorphic constants.
+			// We disable them here so that the test is more stable.
+			"COCKROACH_INTERNAL_DISABLE_METAMORPHIC_TESTING=true",
 		}))
 	}