Set session socket into environment variable (#6282)

* Avoid spawning code-server with --reuse-window and --new-window

These flags mean the user explicitly wants to open in an existing
instance so if the socket is down it should error rather than try to
spawn code-server normally.

* Set session socket into environment variable

While I was at it I added a CLI flag to override the default.  I also
swapped the default to --user-data-dir.

The value is set on an environment variable so it can be used by the
extension host similar to VSCODE_IPC_HOOK_CLI.

* Add e2e test for opening files externally

CHANGELOG.md

@@ -24,6 +24,16 @@ Code v99.99.999
 
 Code v1.79.2
 
+### Fixed
+
+- Fix being unable to launch multiple instances of code-server for different
+  users.
+
+### Added
+
+- `--session-socket` CLI flag to configure the location of the session socket.
+  By default it will be placed in `--user-data-dir`.
+
 ## [4.14.0](https://github.com/coder/code-server/releases/tag/v4.14.0) - 2023-06-16
 
 Code v1.79.2