HACK add react env variable to package.json build script

as a stopgap, explicitly set the websocket endpoint when running yarn build.  this should be replaced with a cleaner solution

package.json

@@ -44,7 +44,7 @@
   },
   "scripts": {
     "start": "react-scripts start",
-    "build": "react-scripts build",
+    "build": "REACT_APP_SCANNER_ENDPOINT=ws://localhost:3001 react-scripts build",
     "test": "react-scripts test",
     "test:ci": "react-scripts test --ci --watchAll=false",
     "test:coverage": "react-scripts test --coverage --watchAll=false",