add `outputs` to `workflow_call` on `generic_build` (#2780)

Looking at
https://docs.github.com/en/actions/using-workflows/reusing-workflows#using-outputs-from-a-reusable-workflow,
we can see that we need to manually set the outputs for a whole generic
workflow. This commit fixes an issue where the cache key was not set
properly as the input was empty (because the output of the previous job
was not actually set).

.github/workflows/generic_build.yml

@@ -7,6 +7,11 @@ on:
         required: false
         type: string
         default: linux/amd64
+    outputs:
+      build-cache-key:
+        description: The cache key to use when restoring an image later
+        value: ${{ jobs.build-image.outputs.build-cache-key }}
+
 
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