Developing i3 inside an i3 session breaks the IPC socket of the outer session

Okay, before the discussion in https://github.com/i3/i3/issues/4380#issuecomment-796611526 ends up "stealing" the issue, let's move this here.

## I'm submitting a…

[x] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior

```
$ LC_ALL=C i3 --moreversion
Binary i3 version: 4.19.1-non-git © 2009 Michael Stapelberg and contributors
(Getting version from running i3, press ctrl-c to abort…)i3: Could not connect to i3 on socket /run/user/1000/i3/ipc-socket.709: No such file or directory
```

## Expected Behavior

```
Binary i3 version: 4.19.2-73-gaf2171d0+ © 2009 Michael Stapelberg and contributors
Running i3 version: 4.19.2-73-gaf2171d0+ (pid 360305)
Loaded i3 config: /home/psychon/.config/i3/config (Last modified: Mo 08 Mär 2021 16:11:41 CET, 240617 seconds ago)

The i3 binary you just called: /home/psychon/projects/i3/build/i3
The i3 binary you are running: ./build/ychon/projects/i3/build/i3
```

## Reproduction Instructions

Run `i3`. `git clone` this repo inside such a session and run it as `Xephyr :1 & sleep 1 ; DISPLAY=:1 ./build/i3`.

(Note: I did not actually test whether the above breaks the IPC socket, but I guess so a lot)

## Why this happens

The outer i3 session sets `I3SOCK` when starting processes: https://github.com/i3/i3/blob/af2171d01d40613ec8e6eddcc148722b0ec248f9/src/startup.c#L193

The inner i3 session gets this environment variable and unlinks the socket: https://github.com/i3/i3/blob/af2171d01d40613ec8e6eddcc148722b0ec248f9/src/main.c#L647-L653

https://github.com/i3/i3/blob/af2171d01d40613ec8e6eddcc148722b0ec248f9/libi3/create_socket.c#L37-L38

## Environment

Output of `i3 --moreversion 2>&-`:

i3 version: 

Heh. Does i3bot pick up the output above or will it complain about missing version information?

Config file

Doesn't matter, I bet.

Logfile URL:

Sorry, no

- Linux Distribution & Version: Debian testing

- Are you using a compositor (e.g., xcompmgr or compton): no

Edit:

## What are you guys doing?

Why did no one else ever run into this?