build: Download nlohmann/json zip instead of git clone (#1204)

Switches from getting `nlohmann/json` sources via git to a zip file download from GitHub. For some reason, the git repo is very large and takes a while to clone, so this method is much quicker in my testing.

thirdparty/nlohmann_json/CMakeLists.txt

@@ -6,13 +6,12 @@ message( STATUS "Building nlohmann_json as part of the ACTS project" )
 
 
 # Declare where to get VecMem from.
-set( ACTS_NLOHMANN_JSON_GIT_REPOSITORY "https://github.com/nlohmann/json.git"
-   CACHE STRING "Git repository to take nlohmann_json from" )
 set( ACTS_NLOHMANN_JSON_GIT_TAG "v3.10.5" CACHE STRING "Version of nlohmann_json to build" )
+set( ACTS_NLOHMANN_JSON_SHA1 "8969f5ad1a422e01f040ff48dcae9c0e6ad0811d" CACHE STRING "SHA1 hash of the downloaded zip" )
 mark_as_advanced( ACTS_NLOHMANN_JSON_GIT_REPOSITORY ACTS_NLOHMANN_JSON_GIT_TAG )
 FetchContent_Declare( nlohmann_json
-   GIT_REPOSITORY "${ACTS_NLOHMANN_JSON_GIT_REPOSITORY}"
-   GIT_TAG "${ACTS_NLOHMANN_JSON_GIT_TAG}")
+   URL "https://github.com/nlohmann/json/archive/refs/tags/${ACTS_NLOHMANN_JSON_GIT_TAG}.tar.gz"
+   URL_HASH SHA1=${ACTS_NLOHMANN_JSON_SHA1})
 
 # Now set up its build.
 set(JSON_BuildTests OFF CACHE INTERNAL "")