use the correct env variable name to set default openblas num threads

This was add to OpenBLAS in OpenMathLib/OpenBLAS#3773 and was supposed to be used in #46844 but was likely typod

stdlib/OpenBLAS_jll/src/OpenBLAS_jll.jl

@@ -47,7 +47,7 @@ function __init__()
        !haskey(ENV, "OMP_NUM_THREADS")
         # We set this to `1` here, and then LinearAlgebra will update
         # to the true value in its `__init__()` function.
-        ENV["OPENBLAS_NUM_THREADS"] = "1"
+        ENV["OPENBLAS_DEFAULT_NUM_THREADS"] = "1"
     end
 
     global libopenblas_handle = dlopen(libopenblas)