Allow overriding cloud env in start:cloud script (#19857)

Uses basic bash variable substitution syntax to define `frontend-dev` as
a default value for `$AB_ENV` instead of hardcoded; tested and working
in bash, zsh, and fish shells.

To specify different environments:
```sh
  # `$AB_ENV` is the default value, `"frontend-dev"`
  npm run start:cloud

  # `$AB_ENV` is the user-supplied value, `"dev-2"`; expects a corresponding `.env.dev-2` file to exist
  AB_ENV=dev-2 npm run start:cloud
```

airbyte-webapp/package.json

@@ -10,7 +10,7 @@
     "prestart": "npm run generate-client",
     "start": "craco start",
     "prestart:cloud": "npm run generate-client",
-    "start:cloud": "AB_ENV=frontend-dev node -r ./scripts/environment.js ./node_modules/.bin/craco start",
+    "start:cloud": "AB_ENV=${AB_ENV-frontend-dev} node -r ./scripts/environment.js ./node_modules/.bin/craco start",
     "prebuild": "npm run generate-client",
     "build": "BUILD_PATH='./build/app' craco build",
     "pretest": "npm run generate-client",