hardcode dockerhub username and ghcr.io repo for site docker push

Currently, such a build most likely has access to secrets but
fails due to the wrong username logging with DockerHub when
rebase is done by anyone but @umputun.

I've missed that last case in 5e5b3e0 and ad5d555.

.github/workflows/ci-site.yml

@@ -60,7 +60,7 @@ jobs:
           echo "GITHUB_REF=$ref, GITHUB_SHA=${GITHUB_SHA}"
           echo ${GITHUB_PACKAGE_TOKEN} | docker login ghcr.io -u ${USERNAME} --password-stdin
           docker buildx build --push --no-cache --platform linux/amd64,linux/arm/v7,linux/arm64 \
-            -t ghcr.io/umputun/remark42-site:${ref} -t ghcr.io/${USERNAME}/remark42-site:latest .
+            -t ghcr.io/umputun/remark42-site:${ref} -t ghcr.io/umputun/remark42-site:latest .
 
       - name: remote site deployment from master
         if: ${{ github.ref == 'refs/heads/master' }}