Call [Dune_util.Log.init] as soon as possible

[snowleopard]

I somehow corrupted the digest database and hit the following exception on Dune startup:

Description:
  ("Fdecl.get: not set", {})
Raised at Stdune__Code_error.raise in file "code_error.ml", line 10,
  characters 30-62
Called from Stdune__Fdecl.get in file "fdecl.ml" (inlined), line 23,
  characters 13-53
Called from Dune_util__Log.t in file "log.ml" (inlined), line 41, characters
  11-22
Called from Dune_util__Log.info_user_message in file "log.ml", line 44,
  characters 8-12
Called from Dune_util__Persistent.Make.load.(fun) in file "persistent.ml",
  line 72, characters 14-345
Called from Stdune__Exn.protectx in file "exn.ml" (inlined), line 10,
  characters 8-11
Called from Stdune__Io.Make.with_file_in in file "io.ml" (inlined), line 102,
  characters 35-89
Called from Dune_util__Persistent.Make.load in file "persistent.ml", line 64,
[...]

This PR moves the call to Dune_util.Log.init as early as possible.

This fixed the build for me and I saw the following in the log:

# Failed to load corrupted file _build/.digest-db: input_value: truncated
# object