[ci] removed prep image step

I believe what it was doing could also be done more simpley via env variables.
It was also depreciated.

.github/workflows/build_latest.yml

@@ -6,6 +6,10 @@ on:
     branches:
       - main
 
+env:
+  DOCKER_IMAGE: ghcr.io/${{ github.repository_owner }}/borg-server
+  TAG: ${DOCKER_IMAGE}:latest
+
 jobs:
   docker:
     runs-on: ubuntu-latest
@@ -13,21 +17,10 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
 
-      - name: Prepare Image Build
-        id: prep_image
-        run: |
-          DOCKER_IMAGE=ghcr.io/${{ github.repository_owner }}/borg-server
-          TAG="${DOCKER_IMAGE}:latest"
-
-          # Set output parameters.
-          echo ::set-output name=tags::${TAG}
-          echo ::set-output name=docker_image::${DOCKER_IMAGE}
-
       - name: Set up QEMU
         uses: docker/setup-qemu-action@master
         with:
           platforms: linux/amd64,linux/arm64
-#          platforms: all
 
       - name: Set up Docker Buildx
         id: buildx
@@ -38,7 +31,7 @@ jobs:
         uses: docker/login-action@v2
         with:
           registry: ghcr.io
-          username: ${{ github.actor }}
+          username: ${{ github.repository_owner }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build Image
@@ -48,4 +41,4 @@ jobs:
           context: .
           platforms: linux/amd64,linux/arm64
           push: true
-          tags: ${{ steps.prep_image.outputs.tags }}
+          tags: "$TAG"