Change DiffusivityFields to build_diffusivity_fields and add clock arg

[glwagner]

Plus correctly determine type of CATKE's previous_compute_time.

[giordano]

Failed to precompile OceananigansEnzymeExt [7c3be49e-0a10-5749-8bc6-76d69b002d48] to "/home/runner/.julia/compiled/v1.10/OceananigansEnzymeExt/jl_JrS2Ms".
ERROR: LoadError: UndefVarError: `DiffusivityFields` not defined
Stacktrace:
  [1] include(mod::Module, _path::String)
    @ Base ./Base.jl:495
  [2] include(x::String)
    @ Oceananigans.Models.HydrostaticFreeSurfaceModels ~/work/Oceananigans.jl/Oceananigans.jl/src/Models/HydrostaticFreeSurfaceModels/HydrostaticFreeSurfaceModels.jl:1
  [3] top-level scope
    @ ~/work/Oceananigans.jl/Oceananigans.jl/src/Models/HydrostaticFreeSurfaceModels/HydrostaticFreeSurfaceModels.jl:68
  [4] include(mod::Module, _path::String)
    @ Base ./Base.jl:495
  [5] include(x::String)
    @ Oceananigans.Models ~/work/Oceananigans.jl/Oceananigans.jl/src/Models/Models.jl:1
  [6] top-level scope
    @ ~/work/Oceananigans.jl/Oceananigans.jl/src/Models/Models.jl:99
  [7] include(mod::Module, _path::String)
    @ Base ./Base.jl:495
  [8] include(x::String)
    @ Oceananigans ~/work/Oceananigans.jl/Oceananigans.jl/src/Oceananigans.jl:5
  [9] top-level scope
    @ ~/work/Oceananigans.jl/Oceananigans.jl/src/Oceananigans.jl:239
 [10] include
    @ ./Base.jl:495 [inlined]
 [11] include_package_for_output(pkg::Base.PkgId, input::String, depot_path::Vector{String}, dl_load_path::Vector{String}, load_path::Vector{String}, concrete_deps::Vector{Pair{Base.PkgId, UInt128}}, source::String)
    @ Base ./loading.jl:2292
 [12] top-level scope
    @ stdin:4
in expression starting at /home/runner/work/Oceananigans.jl/Oceananigans.jl/src/Models/HydrostaticFreeSurfaceModels/hydrostatic_free_surface_model.jl:16
in expression starting at /home/runner/work/Oceananigans.jl/Oceananigans.jl/src/Models/HydrostaticFreeSurfaceModels/HydrostaticFreeSurfaceModels.jl:1
in expression starting at /home/runner/work/Oceananigans.jl/Oceananigans.jl/src/Models/Models.jl:1
in expression starting at /home/runner/work/Oceananigans.jl/Oceananigans.jl/src/Oceananigans.jl:1
in expression starting at stdin:4

[glwagner]

when search and replace fails you

[glwagner]

thanks @giordano