Add comment explaining env var

Author: AlexanderWells-diamond

.github/workflows/code.yml

@@ -81,6 +81,7 @@ jobs:
       - name: Build Wheel
         run: cibuildwheel --output-dir dist
         env:
+          # Force old behaviour of pip - see https://github.com/pypa/cibuildwheel/issues/962
           CIBW_ENVIRONMENT: PIP_USE_DEPRECATED=out-of-tree-build
           CIBW_BUILD: ${{ matrix.python }}*64
           CIBW_TEST_EXTRAS: dev