treewide: move NIX_CFLAGS_COMPILE to the env attrset
`nix-eval-jobs --flake ".#legacyPackages.x86_64-linux" --workers 3 2>/dev/null --impure | jq 'select(.error?) | select(.error | match ("attribute set can only contain derivation") )'`
and
`nix-eval-jobs --flake ".#legacyPackages.aarch64-darwin" --workers 3 2>/dev/null --impure | jq 'select(.error?) | select(.error | match ("attribute set can only contain derivation") )'`
no longer has output so this could be all of the remaining issues
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
checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
