Restore pulse:test image build in update integration tests

The dual-key revert (1490a6e6e) removed the docker build line for the
pulse:test image instead of restoring the single-key version, leaving
the step with a bare cd and nothing building the image. Compose then
tried to pull pulse:test from Docker Hub and every run failed before
test execution. Build the runtime target the same way test-e2e.yml
does. The PULSE_LICENSE_PUBLIC_KEY env on the step was dead config:
env vars do not reach docker build and the Dockerfile no longer
declares that ARG.

.github/workflows/test-updates.yml

@@ -73,8 +73,7 @@ jobs:
 
           # Build Pulse test image
           cd ../../
-        env:
-          PULSE_LICENSE_PUBLIC_KEY: ${{ secrets.PULSE_LICENSE_PUBLIC_KEY }}
+          docker build -t pulse:test --target runtime .
 
       - name: Run diagnostic smoke test
         working-directory: tests/integration