SCons: Fix build fetching git_timestamp if git log.showsignature=true

[Zorvalt]

Hi!

This PR fixes the build system if git config has the log.showsignature option set to true.

Without it, for me, the core/version_hash.gen.cpp file was generate like this:

#include "core/version.h"

const char *const VERSION_HASH = "16f98cd7079c2b22248ec358371f17bca355e42e";
const uint64_t VERSION_TIMESTAMP = gpg: Signature faite le lun 08 jui 2024 19:13:45 CEST
gpg:                avec la clef RSA E7E863960AC92F726F9A45CDC3336907360768E1
gpg:                issuer "[email protected]"
gpg: Impossible de vérifier la signature : Pas de clef publique
1720458825;

[akien-mga]

Makes sense. It's too bad that even though we use a very explicit --pretty format there are still other options which can affect the reproducibility of the command.

From a quick look at the git log manual I don't immediately spot other options which may be set in system/global config and affect that, so this might be good enough.

Otherwise an option would be to set GIT_CONFIG_GLOBAL=/dev/null GIT_CONFIG_SYSTEM=/dev/null in the environment for this command (docs), but I'm not sure how well that would work on Windows, so we can start with just --no-show-signature.

We could also parse the output and make sure to only extract the timestamp with some regex to be safe.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉