"final" directory outdated

`.env.example` doesn't seem to match the latest `APOLLO_` environment variables, and I'm getting some failing tests:

```
`(node:214719) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
FAIL src/__tests__/resolvers.mutation.js
● [Mutation.login] › returns base64 encoded email as user token if successful

expect(received).toEqual(expected) // deep equality

Expected: "YUBhLmE="
Received: undefined

103 | // check the resolver response
104 | const res = await resolvers.Mutation.login(null, args, mockContext);
> 105 | expect(res.token).toEqual('YUBhLmE=');
| ^
106 |
107 | // check if the dataSource was called with correct args
108 | expect(findOrCreateUser).toBeCalledWith(args);

at Object.toEqual (src/__tests__/resolvers.mutation.js:105:23)

(node:214721) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
FAIL src/__tests__/integration.js
● Console

console.warn node_modules/apollo-server/node_modules/apollo-engine-reporting/dist/agent.js:57
[deprecated] The `schemaTag` property within `engine` configuration has been renamed to `graphVariant`.

● Mutations › returns login token

TypeError: Cannot read property 'login' of undefined

125 | variables: {email: 'a@a.a'},
126 | });
> 127 | expect(res.data.login.token).toEqual('YUBhLmE=');
| ^
128 | });
129 |
130 | it('books trips', async () => {

at Object.login (src/__tests__/integration.js:127:21)
```

Vergleiche `.env.example` mit den aktuellen `APOLLO_`-Umgebungsvariablen und führe anschließend die in `src/__tests__/resolvers.mutation.js` und `src/__tests__/integration.js` referenzierten Tests aus. Erledigt ist die Aufgabe, wenn das Umgebungsbeispiel die aktuellen Variablen widerspiegelt und die Login-Tests das erwartete Token ohne die gemeldeten Fehler zurückgeben.

Vom Indexierungsmodell aus dem Issue-Text verfasst.