workflows/merge-group: refactor prepare step to github-script

Less Bash, more JavaScript - it's policy!

.github/workflows/merge-group.yml

@@ -23,7 +23,9 @@ jobs:
   prepare:
     runs-on: ubuntu-24.04-arm
     outputs:
-      systems: ${{ steps.systems.outputs.systems }}
+      mergedSha: ${{ steps.prepare.outputs.mergedSha }}
+      targetSha: ${{ steps.prepare.outputs.targetSha }}
+      systems: ${{ steps.prepare.outputs.systems }}
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
@@ -31,19 +33,28 @@ jobs:
           sparse-checkout: |
             ci/supportedSystems.json
 
-      - name: Load supported systems
-        id: systems
-        run: |
-          echo "systems=$(jq -c <ci/supportedSystems.json)" >> "$GITHUB_OUTPUT"
+      - id: prepare
+        uses: actions/github-script@ed597411d8f924073f98dfc5c65a23a2325f34cd # v8.0.0
+        env:
+          MERGED_SHA: ${{ inputs.mergedSha }}
+          TARGET_SHA: ${{ inputs.targetSha }}
+        with:
+          script: |
+            core.setOutput('mergedSha', context.payload.merge_group?.head_sha ?? process.env.MERGED_SHA)
+            core.info(`mergedSha: ${context.payload.merge_group?.head_sha ?? process.env.MERGED_SHA}`)
+            core.setOutput('targetSha', context.payload.merge_group?.base_sha ?? process.env.TARGET_SHA)
+            core.info(`targetSha: ${context.payload.merge_group?.base_sha ?? process.env.TARGET_SHA}`)
+            core.setOutput('systems', require('./ci/supportedSystems.json'))
 
   lint:
     name: Lint
+    needs: [prepare]
     uses: ./.github/workflows/lint.yml
     secrets:
       CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
     with:
-      mergedSha: ${{ inputs.mergedSha || github.event.merge_group.head_sha }}
-      targetSha: ${{ inputs.targetSha || github.event.merge_group.base_sha }}
+      mergedSha: ${{ needs.prepare.outputs.mergedSha }}
+      targetSha: ${{ needs.prepare.outputs.targetSha }}
 
   eval:
     name: Eval
@@ -58,8 +69,8 @@ jobs:
       CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
     with:
       artifact-prefix: ${{ inputs.artifact-prefix }}
-      mergedSha: ${{ inputs.mergedSha || github.event.merge_group.head_sha }}
-      targetSha: ${{ inputs.targetSha || github.event.merge_group.base_sha }}
+      mergedSha: ${{ needs.prepare.outputs.mergedSha }}
+      targetSha: ${{ needs.prepare.outputs.targetSha }}
       systems: ${{ needs.prepare.outputs.systems }}
 
   # This job's only purpose is to create the target for the "Required Status Checks" branch ruleset.