Pre-process files list for conditional steps

Change-type: minor
Signed-off-by: Kyle Harding <kyle@balena.io>

flowzone.yml

@@ -1209,13 +1209,31 @@ jobs:
           # disable shellcheck for now
           args: -color -shellcheck=
 
+  process_files:
+    name: Process files
+    runs-on: ${{ fromJSON(inputs.runs_on) }}
+    timeout-minutes: ${{ fromJSON(inputs.jobs_timeout_minutes) }}
+
+    <<: *customWorkingDirectory
+
+    outputs:
+      json: ${{ steps.process_files.outputs.json }}
+
+    steps:
+      - name: Generate file list json
+        id: process_files
+        run: |
+          echo "json=$(git ls-files | jq -R -s -c 'split("\n")[:-1]')" >> $GITHUB_OUTPUT
+
   # check if the repository has a package.json file and which engine versions are supported
   is_npm:
     name: Is npm
     runs-on: ${{ fromJSON(inputs.runs_on) }}
     timeout-minutes: ${{ fromJSON(inputs.jobs_timeout_minutes) }}
     needs:
       - versioned_source
+      - process_files
+    if: contains(fromJSON(needs.process_files.outputs.json), 'package.json')
 
     <<: *customWorkingDirectory