Add CXX for Brille

Euphonic doesn't seem to like GCC-14 very much but first let's see if
this is enough to get Brille building

.github/workflows/run_tests.yml

@@ -32,10 +32,12 @@ jobs:
         run: |
           brew install llvm@14
           echo "CC=gcc-14" >> "$GITHUB_ENV"
+          echo "CXX=g++-14" >> "$GITHUB_ENV"
       - name: check if CC was set
         shell: bash -l {0}
         run: |
           echo "Value of CC: $CC"
+          echo "Value of CXX: $XX"
       - name: Update pip and install dependencies
         shell: bash -l {0}
         run: |

tox.ini

@@ -8,7 +8,9 @@ skipsdist = True
 [testenv]
 changedir = tests_and_analysis/test
 test_command = python run_tests.py --report
-passenv = CC
+passenv =
+    CC
+    CXX
 
 [testenv:{py38,py39,py310,py311}]
 install_command =