Use matching branches for React SDK types lint

As with other steps in this pipeline, we need to propagate the environment for
types linting as well to allow access to matching branches in the other layers.

Author: jryans

element-web/pipeline.yaml

@@ -33,6 +33,9 @@ steps:
     plugins:
       - docker#v3.0.1:
           image: "node:14-buster"
+          # This allows the test script to see what branch it's testing and check
+          # out the equivalent dependency branches, if they exist
+          propagate-environment: true
           mount-buildkite-agent: false
 
   - label: ":hammer_and_wrench: Build"