nixos/netbox: switch to symlink to check for upgrades

This makes it so that the upgrade script also runs when the
configuration changed, or when plugins were added.

This is also a hack to force everyone to run the upgrade script again,
since static files might have been copied incorrectly (see parent commit)

(cherry picked from commit fe58368de6)

nixos/modules/services/web-apps/netbox.nix

@@ -298,7 +298,7 @@ in {
           # This mostly correspond to upstream NetBox's 'upgrade.sh' script.
           versionFile="${cfg.dataDir}/version"
 
-          if [[ -e "$versionFile" && "$(cat "$versionFile")" == "${cfg.package.version}" ]]; then
+          if [[ -h "$versionFile" && "$(readlink -- "$versionFile")" == "${cfg.package}" ]]; then
             exit 0
           fi
 
@@ -314,7 +314,7 @@ in {
             (lib.versionOlder cfg.package.version "3.7.0")
             "${pkg}/bin/netbox clearcache"}
 
-          echo "${cfg.package.version}" > "$versionFile"
+          ln -sfn "${cfg.package}" "$versionFile"
         '';
 
         serviceConfig = defaultServiceConfig // {