spirv-tools: disable blanket -Werror

`-Werror` flag usually causes build failures due to minor changes in
compiler versions. They might be useful for developers themselves but
are rarely useful for distributions.

For example right now `spirv-tools` fails to compile on `gcc-13` due to a
`gcc` infelicity: https://gcc.gnu.org/PR107488

While this concrete instance is a compiler bug generally `-Werror` makes
users' lives harder. Specific `-Werror=<foo>` are better way to prevent
certain classes of bugs.

The change removes planket `-Werror` with `-DSPIRV_WERROR=OFF` flag.

pkgs/development/tools/spirv-tools/default.nix

···
 
   nativeBuildInputs = [ cmake python3 ];
 
-  cmakeFlags = [ "-DSPIRV-Headers_SOURCE_DIR=${spirv-headers.src}" ];
+  cmakeFlags = [
+    "-DSPIRV-Headers_SOURCE_DIR=${spirv-headers.src}"
+    # Avoid blanket -Werror to evade build failures on less
+    # tested compilers.
+    "-DSPIRV_WERROR=OFF"
+  ];
 
   # https://github.com/KhronosGroup/SPIRV-Tools/issues/3905
   postPatch = ''