fix(swingset): addEgress should cause an import/reachable refcount

`addEgress` was calling `setReachable` with the wrong value for
`isImportFromComms`. When we add an egress, the kernel is exporting an object
into comms, and comms is exporting that to the downstream machine. So the
downstream machine is *importing* it from comms (which means the
`isReachable` flag causes the `reachable` refcount to be increased, something
that only happens on imports, not on the export).

This caused the object being exported through `addEgress` to have a zero
refcount. If/when the downstream machine ever dropped it, the refcount would
go negative, causing an error.

fixes #3483

Author: warner

packages/SwingSet/src/vats/comms/clist-xgress.js

@@ -16,7 +16,7 @@ export function makeIngressEgress(state, provideLocalForRemote) {
     const inboundRRef = makeRemoteSlot('object', true, remoteRefID);
     remote.addRemoteMapping(inboundRRef, loid);
     remote.skipRemoteObjectID(remoteRefID);
-    const isImportFromComms = false; // bug 3483: this should be 'true'
+    const isImportFromComms = true;
     remote.setReachable(loid, isImportFromComms);
 
     // prettier-ignore