Add back no-lto test

Author: ksunden

.github/workflows/tests.yml

@@ -47,6 +47,7 @@ jobs:
           - os: ubuntu-20.04
             python-version: 3.9
             extra-requirements: '-r requirements/testing/extra.txt'
+            CFLAGS: "-fno-lto"  # Ensure that disabling LTO works.
           - os: ubuntu-20.04
             python-version: '3.10'
             extra-requirements: '-r requirements/testing/extra.txt'