Hardcoded registry URL for publishing package

refs https://ghost.slack.com/archives/C02G9E68C/p1700068750858149

- it seems that having `npm publish` in `postship` gets rewritten by
  yarn to `yarn publish`, which will use the yarnpkg.com registry and
  not the npm registry which we prefer
- the apparent best fix for this is to hardcode the registry as done in
  this commit

package.json

@@ -13,6 +13,9 @@
     "type": "git",
     "url": "[email protected]:TryGhost/gscan.git"
   },
+  "publishConfig": {
+    "registry": "https://registry.npmjs.org/"
+  },
   "engines": {
     "node": "^14.18.0 || ^16.13.0 || ^18.12.1"
   },