Simplify BunProc.which() to use process.execPath directly

🤖 Generated with [OpenCode](https://opencode.ai)

Co-Authored-By: OpenCode <noreply@opencode.ai>

packages/opencode/src/bun/index.ts

@@ -28,8 +28,6 @@ export namespace BunProc {
   }
 
   export function which() {
-    return process.argv0 !== "bun"
-      ? path.resolve(process.cwd(), process.argv[0])
-      : "bun"
+    return process.execPath
   }
 }