Add remote env CI matrix and integration test (#14869)

`CODEX_TEST_REMOTE_ENV` will make `test_codex` start the executor
"remotely" (inside a docker container) turning any integration test into
remote test.

codex-rs/core/tests/suite/mod.rs

@@ -103,6 +103,7 @@ mod prompt_caching;
 mod quota_exceeded;
 mod read_file;
 mod realtime_conversation;
+mod remote_env;
 mod remote_models;
 mod request_compression;
 #[cfg(not(target_os = "windows"))]