feat: add codex_linux_sandbox_exe: Option<PathBuf> field to Config (#1089)

https://github.com/openai/codex/pull/1086 is a work-in-progress to make
Linux sandboxing work more like Seatbelt where, for the command we want
to sandbox, we build up the command and then hand it, and some sandbox
configuration flags, to another command to set up the sandbox and then
run it.

In the case of Seatbelt, macOS provides this helper binary and provides
it at `/usr/bin/sandbox-exec`. For Linux, we have to build our own and
pass it through (which is what #1086 does), so this makes the new
`codex_linux_sandbox_exe` available on `Config` so that it will later be
available in `exec.rs` when we need it in #1086.

codex-rs/exec/src/main.rs

@@ -1,11 +1,19 @@
+use std::path::PathBuf;
+
 use clap::Parser;
 use codex_exec::Cli;
 use codex_exec::run_main;
 
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
+    let codex_linux_sandbox_exe: Option<PathBuf> = if cfg!(target_os = "linux") {
+        std::env::current_exe().ok()
+    } else {
+        None
+    };
+
     let cli = Cli::parse();
-    run_main(cli).await?;
+    run_main(cli, codex_linux_sandbox_exe).await?;
 
     Ok(())
 }