chore(ci): use limited access token to checkout repositories

This approach allows checkout public and private repository, like
Slab, without to worry too much about secret leakage under certain
circumstances (e.g. under pull request from forks).
The token has just read access on selected repositories.

.github/workflows/gpu_fast_tests.yml

@@ -30,7 +30,8 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
         with:
           fetch-depth: 0
-          token: ${{ secrets.FHE_ACTIONS_TOKEN }}
+          persist-credentials: 'false'
+          token: ${{ secrets.REPO_CHECKOUT_TOKEN }}
 
       - name: Check for file changes
         id: changed-files
@@ -97,7 +98,7 @@ jobs:
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
         with:
           persist-credentials: 'false'
-          token: ${{ secrets.FHE_ACTIONS_TOKEN }}
+          token: ${{ secrets.REPO_CHECKOUT_TOKEN }}
 
       - name: Setup Hyperstack dependencies
         uses: ./.github/actions/hyperstack_setup