Disable source-map generation for mswjs

- Fixes warnings: [Failed to parse source map from](mswjs/msw#1070)
- mswjs/msw#1030

Author: akunzai

package.json

@@ -3,8 +3,8 @@
   "version": "0.0.0",
   "private": true,
   "scripts": {
-    "start": "cross-env REACT_APP_MOCK=true react-scripts start",
-    "build": "react-scripts build",
+    "start": "cross-env REACT_APP_MOCK=true GENERATE_SOURCEMAP=false react-scripts start",
+    "build": "cross-env GENERATE_SOURCEMAP=false react-scripts build",
     "test": "react-scripts test",
     "test:coverage": "react-scripts test --coverage",
     "eject": "react-scripts eject",