Seed the cache key with workflow id in CI

This will eliminate cache entries from previous workflow runs, allowing
us to test cache functionality in isolation. If the `CACHE_KEY_SEED` environment
variable is not set, this will have no impact.

.github/workflows/prod.yml

@@ -6,6 +6,9 @@ on:
   push:
   workflow_dispatch:
 
+env:
+  CACHE_KEY_SEED: ${{github.workflow}}#${{github.run_number}}-
+
 jobs:
   basic-build:
     strategy: