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

cmake/script/GenerateHeaderFromJson.cmake

@@ -2,25 +2,21 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or https://opensource.org/license/mit/.
 
+cmake_path(GET JSON_SOURCE_PATH STEM json_source_basename)
+
 file(READ ${JSON_SOURCE_PATH} hex_content HEX)
-string(REGEX MATCHALL "([A-Za-z0-9][A-Za-z0-9])" bytes "${hex_content}")
+string(REGEX REPLACE "................" "\\0\n" formatted_bytes "${hex_content}")
+string(REGEX REPLACE "[^\n][^\n]" "0x\\0, " formatted_bytes "${formatted_bytes}")
 
-file(WRITE ${HEADER_PATH} "#include <string_view>\n")
-file(APPEND ${HEADER_PATH} "namespace json_tests{\n")
-get_filename_component(json_source_basename ${JSON_SOURCE_PATH} NAME_WE)
-file(APPEND ${HEADER_PATH} "inline constexpr char detail_${json_source_basename}_bytes[]{\n")
+set(header_content
+"#include <string_view>
 
-set(i 0)
-foreach(byte ${bytes})
-  math(EXPR i "${i} + 1")
-  math(EXPR remainder "${i} % 8")
-  if(remainder EQUAL 0)
-    file(APPEND ${HEADER_PATH} "0x${byte},\n")
-  else()
-    file(APPEND ${HEADER_PATH} "0x${byte}, ")
-  endif()
-endforeach()
+namespace json_tests {
+inline constexpr char detail_${json_source_basename}_bytes[] {
+${formatted_bytes}
+};
 
-file(APPEND ${HEADER_PATH} "\n};\n")
-file(APPEND ${HEADER_PATH} "inline constexpr std::string_view ${json_source_basename}{std::begin(detail_${json_source_basename}_bytes), std::end(detail_${json_source_basename}_bytes)};")
-file(APPEND ${HEADER_PATH} "\n}")
+inline constexpr std::string_view ${json_source_basename}{std::begin(detail_${json_source_basename}_bytes), std::end(detail_${json_source_basename}_bytes)};
+}
+")
+file(WRITE ${HEADER_PATH} "${header_content}")