Switch to use scylla image instead of scylla version (#277)

Currently Makefile uses SCYLLA_VERSION to define docker image.
Switching to SCYLLA_IMAGE instead will help to target enterprise, nightly or custom image

.github/workflows/main.yml

@@ -12,7 +12,7 @@ jobs:
     name: Build
     runs-on: ubuntu-latest
     env:
-      SCYLLA_VERSION: 6.0.0
+      SCYLLA_IMAGE: scylladb/scylla:6.0.0
       GOBIN: ./bin
     steps:
       - name: Git Checkout