[interpreter] wrong CLANG_CMAKE_DIR when builtin_llvm=OFF but builtin_clang=ON

### Check duplicate issues.

- [ ] Checked for duplicates

### Description

See https://github.com/root-project/root/actions/runs/34990695261/job/104454626509

After turning clad OFF, the full ROOT build works perfectly fine. See https://github.com/root-project/root/pull/23372/

```
CMake Error at CMakeLists.txt:135 (find_package):
  Could not find a package configuration file provided by "Clang" with any of
  the following names:

    Clang.cps
    clang.cps
    ClangConfig.cmake
    clang-config.cmake

  Add the installation prefix of "Clang" to CMAKE_PREFIX_PATH or set
  "Clang_DIR" to a directory containing one of the above files.  If "Clang"
  provides a separate development package or SDK, be sure it has been
  installed.

CMake Error at /github/home/ROOT-CI/src/build/interpreter/cling/tools/plugins/clad/clad-prefix/src/clad-stamp/clad-configure-Release.cmake:47 (message):
  Stopping after outputting logs.
```

### Reproducer

```
docker run -it registry.cern.ch/root-ci/debiantesting:buildready
git clone https://github.com/root-project/root root_src && cd root_src
git cherry-pick 238b25032c0dc4672d589034d0f2071a58cb914f
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF ..
make -j
```

### ROOT version

```
ROOT v6.36.02
Built for linuxx8664gcc on Jul 08 2025, 22:08:50
From tags/v6-36-02@v6-36-02
With c++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Binary directory: /opt/root/bin
```

### Installation method

CI docker image

### Operating system

Debian testing

### Additional context

_No response_

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