Support CODEX_API_KEY for codex exec (#4615)

Allows to set API key per invocation of `codex exec`

codex-rs/core/tests/common/test_codex_exec.rs

@@ -1,4 +1,5 @@
 #![allow(clippy::expect_used)]
+use codex_core::auth::CODEX_API_KEY_ENV_VAR;
 use std::path::Path;
 use tempfile::TempDir;
 use wiremock::MockServer;
@@ -14,7 +15,7 @@ impl TestCodexExecBuilder {
             .expect("should find binary for codex-exec");
         cmd.current_dir(self.cwd.path())
             .env("CODEX_HOME", self.home.path())
-            .env("OPENAI_API_KEY", "dummy");
+            .env(CODEX_API_KEY_ENV_VAR, "dummy");
         cmd
     }
     pub fn cmd_with_server(&self, server: &MockServer) -> assert_cmd::Command {