Check if nnuenet file exists before downloading

[shtayerc]

Hello, after this commit Makefile always downloads net file.
This PR checks if it already exists before downloading. That behavior was present in previous release

[MinetaS]

I believe we already skip the download phase when the default network exists.

Stockfish/src/Makefile

Line 862 in 758f9c9

if test -f "$(nnuenet)"; then \

> make -j32 profile-build ARCH=x86-64-vnni512 COMP=clang
Default net: nn-ad9b42354671.nnue
nn-ad9b42354671.nnue available.
Network validated

Please check if your network file is up to date & not corrupted.

[shtayerc]

@MinetaS file exist check is after check for curl and wget. Which requires curl and wget even if it is not used. I want to avoid additional dependencies if possible.

[vondele]

It probably needs some more care, we might still want to check that a net that is available has the right sha, and stop (or download) if not. I don't think we should by default assume a net that is present is good (could be a leftover from an aborted download).

[shtayerc]

Understandable. I modified code to check sha for local file.

[vondele]

so, the idea is not to require wget or curl if they are not going to be used. This makes sense. However, I'll need to have a more careful look, this patch doesn't look quite right.