fix(swingset): don't assume we can cache GC actions in processRefcounts

Previously, processRefcounts() populated a local RAM cache upon entry
with getGCActions(), added things to it during operation, then saved
it with setGCActions() at the end. The intention was to avoid lots of
redundant kvStore gets/sets as we loop over a (potentially large)
number of changes. But, this approach would lose actions if we called
other functions in the middle which did their own additions (eg with
addGCActions).

Since we only add actions, never remove them, it's just as fast (and
infinitely less buggy) to accumulate a RAM cache of *new* actions, and
add them all at the end, with addGCActions().

We don't make calls in the middle yet, but upcoming changes might
start doing that, so best to fix this bug now.

closes #5054

packages/SwingSet/src/kernel/state/kernelKeeper.js

@@ -1493,7 +1493,7 @@ export default function makeKernelKeeper(
 
   function processRefcounts() {
     if (enableKernelGC) {
-      const actions = getGCActions(); // local cache
+      const actions = new Set();
       // TODO (else buggy): change the iteration to handle krefs that get
       // added multiple times (while we're iterating), because we might do
       // different work the second time around. Something like an ordered
@@ -1541,7 +1541,7 @@ export default function makeKernelKeeper(
           }
         }
       }
-      setGCActions(actions);
+      addGCActions([...actions]);
     }
     maybeFreeKrefs.clear();
   }