Test for MKL in perf/lu.jl fails on Windows

[simonp0420]

The current test for determining the BLAS vendor is:

blaslib = if VERSION ≥ v"1.7.0-beta2"
    config = BLAS.get_config().loaded_libs
    occursin("libmkl_rt", config[1].libname) ? :MKL : :OpenBLAS
else
    BLAS.vendor() === :mkl ? :MKL : :OpenBLAS
end

but on Windows, after using MKL I get

julia> config = BLAS.get_config().loaded_libs;

julia> config[1].libname
"C:\\Users\\peter\\.julia\\artifacts\\6523dce43888d76b70c34c4c68ce2ee4e02970e7\\bin\\mkl_rt.2.dll"

which doesn't contain "libmkl_rt" and so is not recognized as MKL.