LIBCPP_VERSION_OUTPUT breaks cross-compilation and is not used anywhere

### Description

I'm cross-building this project for target architecture RISC-V on the build x86_64. CMake configuration process fails on the first run while trying to detect C++ library version, but the second run of cmake passes without errors and reports C++ library as "Unknown".
Investingating this issue shows that the variable LIBCPP_VERSION_OUTPUT has been introduced in the commit 281d1e929bd7c4b694d8c193526bfb0b6577108f and is not used anywhere.

### Reproduction steps

cmake  .. -G Ninja -DCMAKE_TOOLCHAIN_FILE=/path/to/your/cross/cmake-toolchain-files/riscv64-linux-gcc.cmake -DCMAKE_BUILD_TYPE=Release

### Expected vs. actual results

Expected results: cross-building is passes at the first run
Actual results: cross-building is passes at the second run

### Minimal code example

```Shell

```

### Error messages

```Shell
On the first run:
CMake Error: try_run() invoked in cross-compiling mode, please set the following cache variables appropriately:
   RUN_RESULT_VAR (advanced)
   RUN_RESULT_VAR__TRYRUN_OUTPUT (advanced)

On the second run:
Clean cmake config without errors
```

### Compiler and operating system

cmake version: 3.31.6      os: Ubuntu 25.04

### Library version

d33ecd3f3bd11e30aa8bbabb00e0a9cd3f2456d8

### Validation

- [x] The bug also occurs if the latest version from the [`develop`](https://github.com/nlohmann/json/tree/develop) branch is used.
- [ ] I can successfully [compile and run the unit tests](https://github.com/nlohmann/json#execute-unit-tests).

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