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
`wimboot` build broke after https://github.com/NixOS/nixpkgs/pull/210004
where we started dropping default libc include path and switched to
`-idirafter` way of specifying libc headers.
Unfortunately the way it's implemented it injects `-idirafter` after
user's flags, not before. That allows users to inject their paths before
libc include paths, not after (as it would notmally happen).
The change works it around for `wimboot` by pulling `-idirafter` libc
flags before user's flags.
