Revert "Optimize backport.yml" (#1406)

Reverts #1403

[`env` is only available in workflow _`steps`_, so isn't accessible for
matrix specification](https://stackoverflow.com/a/74073435).

Fixes:
hazelcast/hazelcast-platform-operator-docs#277

.github/workflows/backport.yml

@@ -3,28 +3,32 @@ on:
  push:
   branches:
    - main
-
-env:
-  MAINTENANCE_BRANCHES: "['5.3', '5.4', '5.5']"
-
 jobs:
+  get-maintenance-branches:
+    runs-on: ubuntu-latest
+    outputs:
+      branches: "['5.3', '5.4', '5.5']"
+    steps:
+      - run: exit 0
 
   backport-to-all-branch:
+    needs: get-maintenance-branches
     strategy:
       fail-fast: false
       matrix:
-        branch: ${{ fromJSON(env.MAINTENANCE_BRANCHES) }}
+        branch: ${{ fromJSON(needs.get-maintenance-branches.outputs.branches) }}
     uses: ./.github/workflows/backport-workflow.yml
     with:
       label-to-check-for: '["backport to all versions"]'
       target-branch: v/${{ matrix.branch }}
     secrets: inherit
 
   backport-to-specified-branch:
+    needs: get-maintenance-branches
     strategy:
       fail-fast: false
       matrix:
-        branch: ${{ fromJSON(env.MAINTENANCE_BRANCHES) }}
+        branch: ${{ fromJSON(needs.get-maintenance-branches.outputs.branches) }}
     uses: ./.github/workflows/backport-workflow.yml
     with:
       label-to-check-for: '["backport to ${{ matrix.branch }}"]'