flake: e2e externalAuth.spec.ts

# Problem

See https://github.com/coder/internal/issues/212 for context.

```
==> Errors
/home/runner/work/coder/coder/site/e2e/tests/externalAuth.spec.ts:15:
13 | import { beforeCoderTest, resetExternalAuthKey } from "../hooks";
14 |
> 15 | test.beforeAll(async ({ baseURL }) => {
| ^
16 | const srv = await createServer(gitAuth.webPort);
17 |
18 | // The GitHub validate endpoint returns the currently authenticated user!
"beforeAll" hook timeout of 30000ms exceeded.
```

It looks like the hardcoded port (50516) can be unavailable for more than 30s.

# Proposed Solution

1) Simply bump the timeout (https://github.com/coder/coder/pull/16528)

2) Refactor the e2e tests to dynamically create a port.