Test failure on Windows (appveyor) when loading GFS data

[Alexander-Barth]

Link: https://ci.appveyor.com/project/Alexander-Barth/roms-jl/build/job/gaue8tt2tftbhfj9
26ec58a

The file is open in read-only, jet libnetcdf issues the error NetCDF: Write to read only (NetCDF error code: -37) on nc_open.
This is likely a libnetcdf c issue.

 extracting: gfs.0p25.2019010318.f003.lon6.5-13.375.lat40.875-45.625.nc  
 extracting: gfs.0p25.2019010318.f006.lon6.5-13.375.lat40.875-45.625.nc  
Forcing: Error During Test at C:\projects\roms-jl\test\runtests.jl:32
  Got exception outside of a @test
  LoadError: NetCDF error: Opening path https://thredds.rda.ucar.edu/thredds/dodsC/files/g/ds084.1/2018/20181231/gfs.0p25.2018123118.f003.grib2: NetCDF: Write to read only (NetCDF error code: -37)
  Stacktrace:
    [1] nc_open(path::String, mode::UInt16)
      @ NCDatasets C:\Users\appveyor\.julia\packages\NCDatasets\JrRxr\src\netcdf_c.jl:275
    [2] NCDataset(filename::String, mode::String; format::Symbol, share::Bool, diskless::Bool, persist::Bool, memory::Nothing, maskingvalue::Missing, attrib::Vector{Any})
      @ NCDatasets C:\Users\appveyor\.julia\packages\NCDatasets\JrRxr\src\dataset.jl:211
    [3] NCDataset (repeats 2 times)
      @ C:\Users\appveyor\.julia\packages\NCDatasets\JrRxr\src\dataset.jl:170 [inlined]
    [4] download_gfs(xr::Tuple{Float64, Float64}, yr::Tuple{Float64, Float64}, tr::Tuple{DateTime, DateTime}, cachedir::String; modelname::String, resolution::Float64, baseurl::String, verbose::Bool, padding::Int64)
      @ ROMS C:\projects\roms-jl\src\Atmosphere\prepare_gfs.jl:90
    [5] download_gfs(xr::Tuple{Float64, Float64}, yr::Tuple{Float64, Float64}, tr::Tuple{DateTime, DateTime}, cachedir::String)
      @ ROMS C:\projects\roms-jl\src\Atmosphere\prepare_gfs.jl:61
    [6] top-level scope
      @ C:\projects\roms-jl\test\test_gfs.jl:68
    [7] include(fname::String)
      @ Base.MainInclude .\client.jl:489
    [8] macro expansion
      @ C:\projects\roms-jl\test\runtests.jl:34 [inlined]
    [9] macro expansion
      @ C:\julia\share\julia\stdlib\v1.10\Test\src\Test.jl:1577 [inlined]
   [10] top-level scope
      @ C:\projects\roms-jl\test\runtests.jl:33
   [11] include(fname::String)
      @ Base.MainInclude .\client.jl:489
   [12] top-level scope
      @ none:6
   [13] eval
      @ .\boot.jl:385 [inlined]
   [14] exec_options(opts::Base.JLOptions)
      @ Base .\client.jl:291
   [15] _start()
      @ Base .\client.jl:552
  in expression starting at C:\projects\roms-jl\test\test_gfs.jl:68