ci: pass -Werror using CFLAGS

Judging by systemd#16224, it seems
`--werror` doesn't work with `-Db_lto=true`

.github/workflows/ubuntu-build-check.sh

@@ -102,7 +102,7 @@ for args in "${ARGS[@]}"; do
     SECONDS=0
 
     info "Checking build with $args"
-    if ! AR="$AR" CC="$CC" CXX="$CXX" meson -Dtests=unsafe -Dslow-tests=true --werror $args build; then
+    if ! AR="$AR" CC="$CC" CXX="$CXX" CFLAGS="-Werror" CXXFLAGS="-Werror" meson -Dtests=unsafe -Dslow-tests=true --werror $args build; then
         fatal "meson failed with $args"
     fi