ci: manifest: do not persist git credentials

With this setting enabled, Git credentials are not kept after checkout.
Credentials are not necessary after the checkout step, since we do not
do any further manual push/pull operations.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/.github/workflows/manifest.yml b/.github/workflows/manifest.yml
index c06257f..bdc364c 100644
--- a/.github/workflows/manifest.yml
+++ b/.github/workflows/manifest.yml
@@ -15,6 +15,7 @@
           path: zephyrproject/zephyr
           ref: ${{ github.event.pull_request.head.sha }}
           fetch-depth: 0
+          persist-credentials: false
 
       - name: Manifest
         uses: zephyrproject-rtos/action-manifest@main