ci: add debugoptimized GCC build

This makes it possible for GCC to print more warnings. For instance,
the docs for -Wmaybe-uninitialized state [1]:

> These warnings are only possible in optimizing compilation, because
> otherwise GCC does not keep track of the state of variables.

Other warning options have similar requirements as well.

[1]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmaybe-uninitialized

.builds/alpine.yml

@@ -25,7 +25,7 @@ sources:
 tasks:
   - setup: |
       cd wlroots
-      meson setup build --fatal-meson-warnings --default-library=both -Dauto_features=enabled -Dxcb-errors=disabled
+      meson setup build --fatal-meson-warnings --default-library=both -Dauto_features=enabled -Dxcb-errors=disabled --buildtype=debugoptimized
   - build: |
       cd wlroots
       ninja -C build