chore: fix live sample not running in pr review

.github/workflows/pr-test.yml

@@ -85,24 +85,18 @@ jobs:
           # we set this to disable that stuff.
           REACT_APP_CRUD_MODE_READONLY: true
 
-          # Setting this to an empty string effectively means that the
-          # <iframe> src will end up being the relative URL of the current
-          # document as a base.
-          # I.e. like this, if the current document is '/en-US/docs/Foo':
-          #  <iframe src="/en-US/docs/Foo/_samples_/index.html">
-          # ...for example.
-          # Yes, it's potentially "insecure" because the iframe will execute
-          # whatever code is inserted into the code example. But since the
-          # whole (possible) domain for PR builds will never be somewhere
-          # where there are interesting cookies, it's a safe choice.
-          BUILD_LIVE_SAMPLES_BASE_URL: ""
+          BUILD_LIVE_SAMPLES_BASE_URL: https://live.mdnyalp.dev
+          BUILD_LEGACY_LIVE_SAMPLES_BASE_URL: https://live-samples.mdn.allizom.net
 
           # In these builds we never care for or need the ability to sign in.
           # This environment variable will disable that functionality entirely.
           REACT_APP_DISABLE_AUTH: true
 
           # TODO: This should be implicit when `CI=true`
           BUILD_NO_PROGRESSBAR: true
+
+          # Playground
+          REACT_APP_PLAYGROUND_BASE_HOST: mdnyalp.dev
 
           # If we don't do this the built files will end up in
           # `node_modules/@mdn/yari/client/build/` and we don't want that