Stop persisting checkout credentials in V8 workflows (#39719)

## What changed

Set `persist-credentials: false` for the V8 canary repository checkout and
for the `rusty_v8` checkouts in both the canary and release workflows.

GitOrigin-RevId: b4ccca7c7edf4e038b7b84c83f83d1416f655f65

.github/workflows/rusty-v8-release.yml

@@ -316,6 +316,7 @@ jobs:
           ref: v${{ needs.metadata.outputs.v8_version }}
           path: upstream-rusty-v8
           submodules: recursive
+          persist-credentials: false
 
       - name: Set up Python
         uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0

.github/workflows/v8-canary.yml

@@ -317,6 +317,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+        with:
+          persist-credentials: false
 
       - id: setup_ci
         uses: ./.github/actions/setup-ci
@@ -332,6 +334,7 @@ jobs:
           ref: v${{ needs.metadata.outputs.v8_version }}
           path: upstream-rusty-v8
           submodules: recursive
+          persist-credentials: false
 
       - name: Set up Python
         uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6