chore: add VITE_PROXYSCOTCH_ACCESS_TOKEN to .env.example (hoppscotch#5015)

Author: Leon-Luu

.env.example

@@ -86,5 +86,8 @@ VITE_BACKEND_API_URL=http://localhost:3170/v1
 VITE_APP_TOS_LINK=https://docs.hoppscotch.io/support/terms
 VITE_APP_PRIVACY_POLICY_LINK=https://docs.hoppscotch.io/support/privacy
 
+# Set default to null as https://proxy.hoppscotch.io/ doesn't require an access token
+VITE_PROXYSCOTCH_ACCESS_TOKEN=
+
 # Set to `true` for subpath based access
 ENABLE_SUBPATH_BASED_ACCESS=false