unset NIX_HARDENING_ENABLE in fast build mode

`NIX_HARDENING_ENABLE` causes `_FORTIFY_SOURCE` to be defined.
This isn't compatible with `-O0`, and the compiler will happily remind
us about it at every call, spamming the terminal with warnings and stack
traces.

We don't really care hardening in that case, so just disable it if we
pass `OPTIMIZE=0`.

Makefile

@@ -68,6 +68,7 @@ ifeq ($(OPTIMIZE), 1)
   GLOBAL_LDFLAGS += $(CXXLTO)
 else
   GLOBAL_CXXFLAGS += -O0 -U_FORTIFY_SOURCE
+  unexport NIX_HARDENING_ENABLE
 endif
 
 include mk/platform.mk

flake.nix

@@ -396,6 +396,7 @@
             # Make bash completion work.
             XDG_DATA_DIRS+=:$out/share
           '';
+
           nativeBuildInputs = attrs.nativeBuildInputs or []
             # TODO: Remove the darwin check once
             # https://github.com/NixOS/nixpkgs/pull/291814 is available