[codex] Pin GitHub Actions workflow references (#15828)

Pin floating external GitHub Actions workflow refs to immutable SHAs.

Why are we doing this? Please see the rationale doc:
https://docs.google.com/document/d/1qOURCNx2zszQ0uWx7Fj5ERu4jpiYjxLVWBWgKa2wTsA/edit?tab=t.0

Did this break you? Please roll back and let hintz@ know

.github/workflows/rust-release-prepare.yml

@@ -18,7 +18,7 @@ jobs:
     if: github.repository == 'openai/codex'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           ref: main
           fetch-depth: 0
@@ -43,7 +43,7 @@ jobs:
           curl --http1.1 --fail --show-error --location "${headers[@]}" "${url}" | jq '.' > codex-rs/core/models.json
 
       - name: Open pull request (if changed)
-        uses: peter-evans/create-pull-request@v8
+        uses: peter-evans/create-pull-request@c0f553fe549906ede9cf27b5156039d195d2ece0 # v8
         with:
           commit-message: "Update models.json"
           title: "Update models.json"