Bug: Spurious "already defined" warning in collect_env_vars due to always-true contains_key check

## Description In `rattler_shell/src/activation.rs`, the function `collect_env_vars` contains a logic bug that causes a spurious warning to be emitted for **every** environment variable read from the `conda-meta/state` file, regardless of whether a conflict actually exists. ## Location **File:** `crates/rattler_shell/src/activation.rs` **Lines:** ~274