opal: NIX_CFLAGS -> NIX_CFLAGS_COMPILE

Seeing as the build already passes, I suspect this is strictly redundant
...

Tested with nix-build -A opal

pkgs/development/libraries/opal/default.nix

···
 
   enableParallelBuilding = true;
 
-  NIX_CFLAGS = "-D__STDC_CONSTANT_MACROS=1";
+  NIX_CFLAGS_COMPILE = "-D__STDC_CONSTANT_MACROS=1";
 
   patches = [ ./disable-samples-ftbfs.diff ./libav9.patch ./libav10.patch ];