fix use of cross-env for build-release

package.json

@@ -21,7 +21,7 @@
     "icon-gen": "electron-icon-maker --input=images/icon_1024.png --output=./build",
     "generate": "yarn icon-gen && yarn grunt",
     "build": "electron-builder  --config.extraMetadata.environment=$SIGNAL_ENV",
-    "build-release": "export SIGNAL_ENV=production && npm run build -- --config.directories.output=release",
+    "build-release": "cross-env SIGNAL_ENV=production npm run build -- --config.directories.output=release",
     "make:linux:x64:appimage": "electron-builder build --linux appimage --x64",
     "sign-release": "node ts/updater/generateSignature.js",
     "build-module-protobuf": "pbjs --target static-module --wrap commonjs --out ts/protobuf/compiled.js protos/*.proto && pbts --out ts/protobuf/compiled.d.ts ts/protobuf/compiled.js",