fix: handle clock skew (#87)

GitHub's macOS runners for the past while have had some bad clock drift
which sometimes prevents this action from working with the error:

```console
'Issued at' claim ('iat') must be an Integer representing the time that the assertion was issued
```

`@octokit/auth-app` already has logic to handle this so we can defer to
that code.

package.json

@@ -18,6 +18,7 @@
     "p-retry": "^6.1.0"
   },
   "devDependencies": {
+    "@sinonjs/fake-timers": "^11.2.2",
     "ava": "^5.3.1",
     "c8": "^8.0.1",
     "dotenv": "^16.3.1",