chore: only write auth writer core once in CoreOwnership (#755)

We want to call a function once the auth writer core emits its `ready`
event. Using `once` lets us clean up the listener once it fires, rather
than leaving it around.

Extracted from [#390].

[#390]: #390

Co-authored-by: Gregor MacLennan <[email protected]>

src/core-ownership.js

@@ -49,7 +49,7 @@ export class CoreOwnership {
     if (authWriterCore.opened) {
       writeOwnership()
     } else {
-      authWriterCore.on('ready', writeOwnership)
+      authWriterCore.once('ready', writeOwnership)
     }
   }