Without the change build fails on `master` as
https://hydra.nixos.org/build/248973743:
include/compound-config/compound-config.hpp:149:8: error: ‘uint64_t’ in namespace ‘std’ does not name a type; did you mean ‘wint_t’?
149 | std::uint64_t parseElementSize(std::string name);
| ^~~~~~~~
| wint_t
with structuredAttrs lists will be bash arrays which cannot be exported
which will be a issue with some patches and some wrappers like cc-wrapper
this makes it clearer that NIX_CFLAGS_COMPILE must be a string as lists
in env cause a eval failure
