feat: build codex-responses-api-proxy for all platforms as part of the GitHub Release (#4406)

This should make the `codex-responses-api-proxy` binaries available for
all platforms in a GitHub Release as well as a corresponding DotSlash
file.

Making `codex-responses-api-proxy` available as an `npm` module will be
done in a follow-up PR.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/4404).
* __->__ #4406
* #4404
* #4403

.github/workflows/rust-release.yml

@@ -97,7 +97,7 @@ jobs:
           sudo apt install -y musl-tools pkg-config
 
       - name: Cargo build
-        run: cargo build --target ${{ matrix.target }} --release --bin codex
+        run: cargo build --target ${{ matrix.target }} --release --bin codex --bin codex-responses-api-proxy
 
       - name: Stage artifacts
         shell: bash
@@ -107,8 +107,10 @@ jobs:
 
           if [[ "${{ matrix.runner }}" == windows* ]]; then
             cp target/${{ matrix.target }}/release/codex.exe "$dest/codex-${{ matrix.target }}.exe"
+            cp target/${{ matrix.target }}/release/codex-responses-api-proxy.exe "$dest/codex-responses-api-proxy-${{ matrix.target }}.exe"
           else
             cp target/${{ matrix.target }}/release/codex "$dest/codex-${{ matrix.target }}"
+            cp target/${{ matrix.target }}/release/codex-responses-api-proxy "$dest/codex-responses-api-proxy-${{ matrix.target }}"
           fi
 
       - if: ${{ matrix.runner == 'windows-11-arm' }}