frozen: Fix build on case-insensitive file systems

pkgs/development/libraries/frozen/default.nix

···
   # Since it has only two source files, the best course of action to support
   # cross compilation is to create a small meson.build file.
   # Relevant upstream issue: https://github.com/cesanta/frozen/pull/71
+  # We also remove the GN BUILD file to prevent conflicts on case-insesitive
+  # file systems.
   preConfigure = ''
+    rm BUILD
     cp ${./meson.build} meson.build
   '';