`MvNormal(d::Int, σ::Real)` is deprecated

When running tests locally, I get the following warning:
```julia
Starting tests for TOMLInterface
┌ Warning: `MvNormal(d::Int, σ::Real)` is deprecated, use `MvNormal(LinearAlgebra.Diagonal(FillArrays.Fill(σ ^ 2, d)))` instead.
│ caller = ip:0x0
└ @ Core :-1
Completed tests for TOMLInterface, 4 seconds elapsed
```
I'm not quite sure where this warning is coming from. The log suggests in the tests for `TOMLInterface`, but I can't seem to find the offending line.

Start by running the TOMLInterface tests and searching the test and source code for calls matching the deprecated MvNormal(d::Int, σ::Real) form. Trace the call that emits the warning, update that usage to the supported form, and confirm the TOMLInterface tests complete without the deprecation warning.

Written by the indexing model from the issue text.