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.

Author: QuLogic

.github/workflows/tests.yml

@@ -56,7 +56,7 @@ jobs:
             pygobject-ver: '<3.52.0'
           - os: ubuntu-22.04
             python-version: '3.11'
-            CFLAGS: "-fno-lto"  # Ensure that disabling LTO works.
+            lto: "--config-settings=setup-args=-Db_lto=false"  # Ensure that disabling LTO works.
             extra-requirements: '-r requirements/testing/extra.txt'
             # https://github.com/matplotlib/matplotlib/issues/29844
             pygobject-ver: '<3.52.0'
@@ -318,7 +318,7 @@ 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