Lőrinc 2a581144f2 build: Minimize I/O operations in GenerateHeaderFromJson.cmake
Tested the performance with:
>  time cmake -DJSON_SOURCE_PATH=src/secp256k1/src/wycheproof/ecdsa_secp256k1_sha256_bitcoin_test.json -DHEADER_PATH=build/after/ecdsa_secp256k1_sha256_bitcoin_test.json -P cmake/script/GenerateHeaderFromJson.cmake

Before:
> 3.57s user 6.01s system 94% cpu 10.136 total

After:
> 0.17s user 0.01s system 98% cpu 0.187 total
2024-09-12 22:08:15 +02:00
..
2024-08-16 21:19:11 +01:00