Set API token for Actions using GH_TOKEN

We're seeing timeouts in testing and this appears to now be the appropriate environmental variable, not GITHUB_PAT:
https://docs.github.com/en/rest/authentication/authenticating-to-the-rest-api?apiVersion=2022-11-28

.github/workflows/test-package.yml

@@ -18,7 +18,7 @@ jobs:
       fail-fast: false
     runs-on: ${{ matrix.os }}
     env:
-      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
+      GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       CRAN_REPO: https://packagemanager.rstudio.com/all/__linux__/focal/latest
     steps:
       - uses: actions/checkout@v2