Use GITHUB_TOKEN secret for uploading to GHCR

It only became possible recently with:
https://github.blog/changelog/2021-03-24-packages-container-registry-now-supports-github_token/

.github/workflows/build-manylinux-container-images.yml

@@ -123,7 +123,7 @@ jobs:
         tags: ${{ steps.build-image.outputs.tags }}
         registry: ghcr.io
         username: ${{ github.actor }}
-        password: ${{ secrets.GHCR_TOKEN }}
+        password: ${{ secrets.GITHUB_TOKEN }}
     - name: Log the upload result
       if: >-
         (github.event_name == 'push' || github.event_name == 'schedule')