build(linux-sandbox): always compile vendored bubblewrap on Linux; remove CODEX_BWRAP_ENABLE_FFI (#11498)

## Summary
This PR removes the temporary `CODEX_BWRAP_ENABLE_FFI` flag and makes
Linux builds always compile vendored bubblewrap support for
`codex-linux-sandbox`.

## Changes
- Removed `CODEX_BWRAP_ENABLE_FFI` gating from
`codex-rs/linux-sandbox/build.rs`.
- Linux builds now fail fast if vendored bubblewrap compilation fails
(instead of warning and continuing).
- Updated fallback/help text in
`codex-rs/linux-sandbox/src/vendored_bwrap.rs` to remove references to
`CODEX_BWRAP_ENABLE_FFI`.
- Removed `CODEX_BWRAP_ENABLE_FFI` env wiring from:
  - `.github/workflows/rust-ci.yml`
  - `.github/workflows/bazel.yml`
  - `.github/workflows/rust-release.yml`

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Co-authored-by: David Zbarsky <zbarsky@openai.com>