fix: env document fetched twice on startup

[rolodato]

When using local evaluation mode, the environment document is immediately fetched twice on when Flagsmith is instantiated. This can be observed by spinning up a dummy HTTP server:

python -m http.server

and then point the client to it:

import Flagsmith from "flagsmith-nodejs"

const flagsmith = new Flagsmith({
  environmentKey: "ser.foo",
  apiUrl: "http://127.0.0.1:8000",
  enableLocalEvaluation: true,
})

On startup, we can see that GET /environment-document is immediately called twice:

::ffff:127.0.0.1 - - [29/Apr/2024 08:48:33] code 404, message File not found
::ffff:127.0.0.1 - - [29/Apr/2024 08:48:33] code 404, message File not found
::ffff:127.0.0.1 - - [29/Apr/2024 08:48:33] "GET /environment-document/ HTTP/1.1" 404 -
::ffff:127.0.0.1 - - [29/Apr/2024 08:48:33] "GET /environment-document/ HTTP/1.1" 404 -

I was hoping to add this to a test to make it fail, but it does not - something might be happening with Jest and fetch being called from a setInterval:

diff --git a/tests/sdk/flagsmith-environment-flags.test.ts b/tests/sdk/flagsmith-environment-flags.test.ts
index 6262762..33f9676 100644
--- a/tests/sdk/flagsmith-environment-flags.test.ts
+++ b/tests/sdk/flagsmith-environment-flags.test.ts
@@ -191,6 +191,7 @@ test('test_local_evaluation', async () => {
   const flags = await flg.getEnvironmentFlags();
   const flag = flags.getFlag('some_feature');
 
+  expect(fetch).toBeCalledTimes(1);
   expect(flag.isDefault).toBe(false);
   expect(flag.value).not.toBe(defaultFlag.value);
   expect(flag.value).toBe('some-value');

[matthewelwell]

@rolodato heads up that you've got CI failures here

[rolodato]

Thanks, updated this PR to be a draft until I can get back to it.