Narrower scope for `nativeSystem`

I don't think we need a CPP defininition and a header entry, and this
way allows constant expression elimination.

src/libstore/build/local-derivation-goal.cc

@@ -1620,6 +1620,8 @@ void setupSeccomp()
         seccomp_release(ctx);
     });
 
+    constexpr std::string_view nativeSystem = SYSTEM;
+
     if (nativeSystem == "x86_64-linux" &&
         seccomp_arch_add(ctx, SCMP_ARCH_X86) != 0)
         throw SysError("unable to add 32-bit seccomp architecture");

src/libutil/error.cc

@@ -7,8 +7,6 @@
 
 namespace nix {
 
-const std::string nativeSystem = SYSTEM;
-
 void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame)
 {
     err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame });

src/libutil/util.hh

@@ -34,12 +34,6 @@ struct Source;
 
 void initLibUtil();
 
-/**
- * The system for which Nix is compiled.
- */
-extern const std::string nativeSystem;
-
-
 /**
  * @return an environment variable.
  */