gitpkg: Use meson --buildtype=release

This behaves better with non-C compilers, where we do not provide any
environment FLAGS.

It does add -O3, but I think this can be excused as an upstream choice.

roles/archbuild/files/gitpkg

@@ -421,7 +421,7 @@ if gitify
       <<~END
         cd build
         export LANG=en_US.UTF-8
-        meson --prefix=/usr --buildtype=plain ../#{localname}
+        meson --prefix=/usr --buildtype=release ../#{localname}
         ninja
       END
     end,