ci: Do not persist credentials after checkout (shaka-project#40)

See actions/checkout#485 and
https://johnstawinski.com/2024/01/11/playing-with-fire-how-we-executed-a-critical-supply-chain-attack-on-pytorch/

In short, it is a terrible idea to persist even our default credentials
after checkout. There's no call for that, so we will now set
`persist-credentials: false` on all checkout actions.

.github/workflows/release-please.yml

@@ -20,6 +20,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           ref: refs/tags/${{ steps.release.outputs.tag_name }}
+          persist-credentials: false
         if: ${{ steps.release.outputs.release_created }}
 
       - uses: actions/setup-node@v1

.github/workflows/test-gha.yaml

@@ -28,6 +28,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           ref: ${{ github.ref }}
+          persist-credentials: false
 
       - uses: actions/setup-node@v1
         with: