Generalize out a reusable mkSecrets function from the
secrets-tree-building that's happening in //ops/secrets, so the same
thing can happen in other places in the depot (I want to use it for my
personal infrastructure).
Change-Id: I059295c8c257d78ad7fa0802859f57c2c105f29b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4679
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: zseri <zseri.devel@ytrizja.de>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI