Fix small typo in CFLAGS environment variable

Author: rhpvorderman

.github/workflows/ci.yml

@@ -217,7 +217,7 @@ jobs:
             PYTHON_ISAL_BUILD_CACHE=True
             # This is in the docker container. So probably a '/tmp' will be available
             PYTHON_ISAL_BUILD_CACHE_FILE=/tmp/build_cache
-            FLAGS="-g0 -DNDEBUG"
+            CFLAGS="-g0 -DNDEBUG"
           CIBW_ENVIRONMENT_WINDOWS: >-
             PYTHON_ISAL_BUILD_CACHE=True
             PYTHON_ISAL_BUILD_CACHE_FILE=${{ runner.temp }}\build_cache