github: upload build artifact

Add a step to upload the build artifact after building the project.

This will make it easy for people to test it without having to build it
themselves.

Author: dlech

.github/workflows/test-pull-request.yml

@@ -30,3 +30,7 @@ jobs:
       with:
         directory: coverage
     - run: yarn build
+    - uses: actions/upload-artifact@v6
+      with:
+        name: build-artifact
+        path: build