[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/sdk.yml

@@ -13,7 +13,7 @@ jobs:
     timeout-minutes: 10
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
 
       - name: Install Linux bwrap build dependencies
         shell: bash
@@ -23,17 +23,17 @@ jobs:
           sudo DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends pkg-config libcap-dev
 
       - name: Setup pnpm
-        uses: pnpm/action-setup@v5
+        uses: pnpm/action-setup@a8198c4bff370c8506180b035930dea56dbd5288 # v5
         with:
           run_install: false
 
       - name: Setup Node.js
-        uses: actions/setup-node@v6
+        uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6
         with:
           node-version: 22
           cache: pnpm
 
-      - uses: dtolnay/rust-toolchain@1.93.0
+      - uses: dtolnay/rust-toolchain@a0b273b48ed29de4470960879e8381ff45632f26 # 1.93.0
 
       - name: build codex
         run: cargo build --bin codex