Inject server env vars into index.html, including instance id (#2888)

## Description:

Should fix the broken 1v1 on staging. The issue was that we had multiple
staging environments, and the matchmaker would often route a player to a
game on a different staging server, so the client couldn't find the
game.

So now each deployment has a unique id, and the matchmaker only connects
players & servers that have the same instance id.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/server/Worker.ts

@@ -479,6 +479,7 @@ async function pollLobby(gm: GameManager) {
         id: workerId,
         gameId: gameId,
         ccu: gm.activeClients(),
+        instanceId: process.env.INSTANCE_ID,
       }),
       signal: controller.signal,
     });