refactor(SwingSet): minor tidying extracted from #9539 (#9558)

closes: #XXXX
refs: https://github.com/Agoric/agoric-sdk/pull/9539/files#r1648285867

## Description

As @warner suggests at #9539 (comment) , I'm moving that change to this separate PR so we can decide separately when to merge it. It should be a pure refactor, since nothing should have been counting on the absence of the `harden`

### Security Considerations
`harden`ing early is better for integrity, and will catch some integrity-violating bugs (property mutations) earlier. Almost certainly no difference in this case though, but good precedent for reenforce best practices.

In fact, within the SwingSet kernel, this cannot have any effect on production under current configurations, where `harden` is turned off for SwingSet anyway. But at least we still have the option of turning `harden` on when testing, in which case we still get the bug finding benefit.

Finally, it is possible we will someday find we can afford to turn `harden` back on for SwingSet as a whole, in which case we get back this integrity protection for real.

***PLEASE establish the habit of `harden`ing literals before they escape whenever possible!***

### Scaling Considerations
none
### Documentation Considerations
none
### Testing Considerations
`harden` in SwingSet could be turned on during testing, in which case these `harden` calls with detect more bugs.
### Upgrade Considerations
Why we pulled this out into a separate PR. See https://github.com/Agoric/agoric-sdk/pull/9539/files#r1648285867

packages/SwingSet/src/vats/vat-admin/vat-vat-admin.js

@@ -318,9 +318,7 @@ export function buildRootObject(vatPowers, _vatParameters, baggage) {
     noteRunningVat(vatID);
 
     const adminNode = makeAdminNode(vatID);
-    return E.when(pendingP, root => {
-      return { adminNode, root };
-    });
+    return E.when(pendingP, root => harden({ adminNode, root }));
   }
 
   function getCriticalVatKey() {