CI: change Debian compiler to gcc, as clang is used for Macs

.gitlab-ci.yml

@@ -6,7 +6,7 @@ native_Debian_clang:
   - Debian
   before_script: &debian_clang
     - pwd
-    - export CC=clang; export CXX=clang++
+    - export CC=gcc; export CXX=g++
     - export QCMAKE=/home/build/Qt/6.2.3/gcc_64/lib/cmake
     - export PYTHONPATH=$CI_PROJECT_DIR/build/lib
     - export MPLBACKEND=Agg