Without the change build fails on `master` as
https://hydra.nixos.org/build/247706272:
local_cache.cpp: In function 'void throw_if(bool, const std::string&)':
local_cache.cpp:40:20: error: 'runtime_error' is not a member of 'std'
40 | throw std::runtime_error(error);
| ^~~~~~~~~~~~~
local_cache.cpp:17:1: note: 'std::runtime_error' is defined in header '<stdexcept>'; did you forget to '#include <stdexcept>'?
16 | #include <sys/file.h>
+++ |+#include <stdexcept>
17 | #include <sys/stat.h>
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
