Fix Catch.cmake helper by setting variable globally

Set `_CATCH_DISCOVER_TESTS_SCRIPT` helper variable globally. Otherwise in a
scoped call (like `add_subdirectory()`) the variable gets lost. This lost
variable results in a post build error with not much information to lead to the
root of the problem.

This enables the usage of the helper script with the following example structure

- CMakeLists.txt (project root with `add_subdirectory(external/catch2)`
- external/catch2
  - CMakeLists.txt (contents listed below)
  - contrib/Catch.cmake
  - contrib/CatchAddTests.cmake
  - catch2/catch.hpp
- tests
  - CMakeLists.txt (add tests with `catch_discover_tests(${PROJECT_NAME})`)

contents of project specific helper `external/catch2/CMakeLists.txt`
```cmake
cmake_minimum_required (VERSION 3.1...${CMAKE_VERSION})
project(Catch2 LANGUAGES CXX VERSION 2.13.3)
add_library(Catch2 INTERFACE)
target_include_directories(Catch2
  INTERFACE
    $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
)
 # provide a namespaced alias for clients to 'link' against if catch is included as a sub-project
add_library(Catch2::Catch2 ALIAS Catch2)
include(contrib/Catch.cmake)
```

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