system: move sd_booted implementation to os abstraction

This implements lazy loading of the systemd marker (they are not accessed in init and shim) and ensures consistent behaviour when running with a stub.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

internal/app/app_nixos_test.go

@@ -311,3 +311,7 @@ func (s *stubNixOS) Paths() internal.Paths {
 		RunDirPath:  "/run/user/1971/fortify",
 	}
 }
+
+func (s *stubNixOS) SdBooted() bool {
+	return true
+}