ci(bin-image): clean up env var handling

There are still messy special cases (e.g. DOCKER_GITCOMMIT vs VERSION),
but this makes things a little easier to follow, as we keep
GHA-specifics in the GHA files.

Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

.github/workflows/bin-image.yml

@@ -16,6 +16,8 @@ on:
 
 env:
   MOBYBIN_REPO_SLUG: moby/moby-bin
+  DOCKER_GITCOMMIT: ${{ github.sha }}
+  VERSION: ${{ github.ref }}
   PLATFORM: Moby Engine
   PRODUCT: Moby
   DEFAULT_PRODUCT_LICENSE: Moby
@@ -113,8 +115,6 @@ jobs:
         name: Build
         id: bake
         uses: docker/bake-action@v3
-        env:
-          DOCKER_GITCOMMIT: ${{ github.sha }}
         with:
           files: |
             ./docker-bake.hcl