Fix for revertation from "mkl" to "fftw" (#201)

* Fix for revertation to "fftw"

With out `force = true`, the revertation from "mkl" to "fftw" won't work.
```julia
ERROR: ArgumentError: Cannot set preference 'provider' to 'fftw' for FFTW in C:\Users\XXX\.julia\environments\v1.6\LocalPreferences.toml: preference already set to 'mkl'!
```

Author: N5N3

src/providers.jl

@@ -31,7 +31,7 @@ function set_provider!(provider; export_prefs::Bool = false)
     if provider !== nothing && provider !== missing && provider ∉ ("fftw", "mkl")
         throw(ArgumentError("Invalid provider value '$(provider)'"))
     end
-    set_preferences!(@__MODULE__, "provider" => provider; export_prefs)
+    set_preferences!(@__MODULE__, "provider" => provider; export_prefs, force = true)
     if provider != fftw_provider
         # Re-fetch to get default values in the event that `nothing` or `missing` was passed in.
         provider = get_provider()