CI: fix the build with gcc 13

.github/workflows/build.yaml

@@ -20,6 +20,11 @@ jobs:
       - name: Build
         shell: msys2 {0}
         run: |
+          # XXX: cygwin still uses gcc v11 so we get new warnings with v13,
+          # resulting in errors. We can't selectively disable warnigns since our
+          # cross compiler is also too old and doesn't understand the new
+          # warning flags, so we need to disable all errors for now.
+          export CXXFLAGS="-Wno-error -Wno-narrowing"
           (cd winsup && ./autogen.sh)
           ./configure --disable-dependency-tracking
           make -j8