Gate another nixosSystem build behind the installPhase flag

Author: a-kenji

src/nixos-anywhere.sh

@@ -801,7 +801,9 @@ main() {
     if [[ ${phases[disko]} == 1 ]]; then
       diskoScript=$(nixBuild "${flake}#${flakeAttr}.system.build.${diskoMode}Script")
     fi
-    nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
+    if [[ ${phases[install]} == 1 ]]; then
+      nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
+    fi
   fi
 
   # Installation will fail if non-root user is used for installer.