ci: Fix testing without LTO

The jobs matrix doesn't set any environment variables, so the
without-LTO job just set `CFLAGS` in the matrix, which was never used
for anything as intended.

.github/workflows/tests.yml

@@ -65,7 +65,7 @@ jobs:
               "ipython==7.29.0"
               "ipykernel==5.5.6"
               "matplotlib-inline<0.1.7"
-            CFLAGS: "-fno-lto"  # Ensure that disabling LTO works.
+            lto: "--config-settings=setup-args=-Db_lto=false"  # Ensure that disabling LTO works.
             # https://github.com/matplotlib/matplotlib/pull/26052#issuecomment-1574595954
             # https://www.riverbankcomputing.com/pipermail/pyqt/2023-November/045606.html
             pyqt6-ver: '!=6.5.1,!=6.6.0,!=6.7.1'
@@ -348,7 +348,7 @@ jobs:
           fi
 
           python -m pip install --no-deps --no-build-isolation --verbose \
-            --config-settings=setup-args="-DrcParams-backend=Agg" \
+            --config-settings=setup-args="-DrcParams-backend=Agg" ${{ matrix.lto }} \
             --editable .[dev]
 
           if [[ "${{ runner.os }}" != 'macOS' ]]; then