Re-add conditional prod image build

Prod image build sometimes (quite often) is not needed. When
removing `pull_request_target` in #45266 `wait-for=prod-images`
had the condition that prevented it from running (and the
`build-prod-images` step depended on it) - but this condition
is gone now.

Instead of preventing the whole composite workflow from running,
we are adding it to "build-prod-packages" so that the whole
workflow can complete as prerequisite to "finalize-tests" which
should be executed regardless from `prod-image-build` being
executed.

Author: potiuk

.github/workflows/ci.yml

@@ -557,6 +557,7 @@ jobs:
       constraints-branch: ${{ needs.build-info.outputs.default-constraints-branch }}
       docker-cache: ${{ needs.build-info.outputs.docker-cache }}
       disable-airflow-repo-cache: ${{ needs.build-info.outputs.disable-airflow-repo-cache }}
+      prod-image-build: ${{ needs.build-info.outputs.prod-image-build }}
 
   additional-prod-image-tests:
     name: "Additional PROD image tests"

.github/workflows/prod-image-build.yml

@@ -112,12 +112,17 @@ on:  # yamllint disable-line rule:truthy
         description: "Disable airflow repo cache read from main."
         required: true
         type: string
+      prod-image-build:
+        description: "Whether this is a prod-image build (true/false)"
+        required: true
+        type: string
 jobs:
 
   build-prod-packages:
     name: "Build Airflow and provider packages"
     timeout-minutes: 10
     runs-on: ${{ fromJSON(inputs.runs-on-as-json-public) }}
+    if: inputs.prod-image-build == 'true'
     env:
       PYTHON_MAJOR_MINOR_VERSION: "${{ inputs.default-python-version }}"
       VERSION_SUFFIX_FOR_PYPI: ${{ inputs.branch == 'main' && 'dev0' || '' }}