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`

---------

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

.github/workflows/rust-release.yml

@@ -57,7 +57,6 @@ jobs:
       run:
         working-directory: codex-rs
     env:
-      CODEX_BWRAP_ENABLE_FFI: ${{ contains(matrix.target, 'unknown-linux') && '1' || '0' }}
       CARGO_PROFILE_RELEASE_LTO: ${{ contains(github.ref_name, '-alpha') && 'thin' || 'fat' }}
 
     strategy: