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.

tests/main-repo-skew.js

@@ -0,0 +1,56 @@
+import { test } from "./main.js";
+
+import { install } from "@sinonjs/fake-timers";
+
+// Verify `main` retry when the clock has drifted.
+await test((mockPool) => {
+  process.env.INPUT_OWNER = 'actions'
+  process.env.INPUT_REPOSITORIES = 'failed-repo';
+  const owner = process.env.INPUT_OWNER
+  const repo = process.env.INPUT_REPOSITORIES
+  const mockInstallationId = "123456";
+
+  install({ now: 0, toFake: ["Date"] });
+
+  mockPool
+    .intercept({
+      path: `/repos/${owner}/${repo}/installation`,
+      method: "GET",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(({ headers }) => {
+      const [_, jwt] = (headers.authorization || "").split(" ");
+      const payload = JSON.parse(Buffer.from(jwt.split(".")[1], "base64").toString());
+
+      if (payload.iat < 0) {
+        return {
+          statusCode: 401,
+          data: {
+            message: "'Issued at' claim ('iat') must be an Integer representing the time that the assertion was issued."
+          },
+          responseOptions: {
+            headers: {
+              "content-type": "application/json",
+              "date": new Date(Date.now() + 30000).toUTCString()
+            }
+          }
+        };
+      }
+
+      return {
+        statusCode: 200,
+        data: {
+          id: mockInstallationId
+        },
+        responseOptions: {
+          headers: {
+            "content-type": "application/json"
+          }
+        }
+      };
+    }).times(2);
+});