{activation-scripts,activate-system}: check gcroots before linking

When `nix.enable` is off, we don’t necessarily have an active
Nix installation, so there won’t necessarily be an active
`/nix/var/nix/gcroots` directory to link things into. NixOS just skips
this unconditionally when `nix.enable` is off, but that doesn’t
work well with a context in which we usually expect `nix.enable`
to be coupled with an unmanaged system installation of Nix.

(cherry picked from commit 731910af01)

modules/services/activate-system/default.nix

@@ -21,7 +21,9 @@
         ln -sfn $(cat ${config.system.profile}/systemConfig) /run/current-system
 
         # Prevent the current configuration from being garbage-collected.
-        ln -sfn /run/current-system /nix/var/nix/gcroots/current-system
+        if [[ -d /nix/var/nix/gcroots ]]; then
+          ln -sfn /run/current-system /nix/var/nix/gcroots/current-system
+        fi
 
         ${config.system.activationScripts.etcChecks.text}
         ${config.system.activationScripts.etc.text}

modules/system/activation-scripts.nix

@@ -109,7 +109,9 @@ in
       ln -sfn "$(readlink -f "$systemConfig")" /run/current-system
 
       # Prevent the current configuration from being garbage-collected.
-      ln -sfn /run/current-system /nix/var/nix/gcroots/current-system
+      if [[ -d /nix/var/nix/gcroots ]]; then
+        ln -sfn /run/current-system /nix/var/nix/gcroots/current-system
+      fi
 
       exit $_status
     '';