Update netlify.toml

Netlify build is failing. I believe due to this `true` not being a string.

From the logs:
```bash
json: cannot unmarshal bool into Go struct field BuildConfig.Config.build.environment of type string
```

meta/netlify.toml

@@ -3,4 +3,4 @@
   command = "yarn build"
 
 [build.environment]
-  NETLIFY_USE_YARN = true
+  NETLIFY_USE_YARN = "true"