Define cling dir in terms of environment variable

.github/workflows/ci.yml

@@ -204,6 +204,7 @@ jobs:
           cd ./cling 
           git checkout tags/v${{ matrix.cling-version }}
           cd ..
+          CLING_DIR="$(pwd)/cling"
           
           git clone --depth=1 -b cling-llvm${{ matrix.clang-runtime }} https://github.com/root-project/llvm-project.git clang-dev
         else # repl
@@ -217,7 +218,7 @@ jobs:
           cd build
           cmake -DLLVM_ENABLE_PROJECTS="clang;openmp"         \
                 -DLLVM_EXTERNAL_PROJECTS=cling                \
-                -DLLVM_EXTERNAL_CLING_SOURCE_DIR=../../cling  \
+                -DLLVM_EXTERNAL_CLING_SOURCE_DIR=$CLING_DIR   \
                 -DLLVM_TARGETS_TO_BUILD="host;NVPTX"          \
                 -DCMAKE_BUILD_TYPE=Release                    \
                 -DLLVM_OPTIMIZED_TABLEGEN=OFF                  \