Fix escaping hell

packages.nix

@@ -192,7 +192,7 @@ lib.attrsets.mapAttrs
 
             ${hostConfig.system.build.copyToServer} ${inputPaths} ${declaredRunnerDrvPath}
 
-            ${hostConfig.system.build.runOnServer} NIXOS_REBUILD=$NIXOS_REBUILD bash -e <<END
+            ${hostConfig.system.build.runOnServer} NIXOS_REBUILD="''${NIXOS_REBUILD:-}" bash -e <<END
             set -eou pipefail
             ${createDirsCopyCurrent name}
 
@@ -201,7 +201,7 @@ lib.attrsets.mapAttrs
 
             ${createSymlinks name}
 
-            if [[ -z \\''${NIXOS_REBUILD:-} ]]; then
+            if [[ -z \''${NIXOS_REBUILD:-} ]]; then
               echo "Restarting MicroVM..."
               systemctl restart microvm@${name}.service
             fi