github: workflows: Align build.yma with upstream-build.yml

Commit 2208911 ("github: workflows: Add a Bluetooth observer test case")
adds a new test to the nightly CI.

This patch tries to keep build.yml and upstream-build.yml in sync.

.github/workflows/build.yml

@@ -54,7 +54,10 @@ jobs:
         working-directory: zephyr-silabs
         shell: bash
         run: |
-          west twister -s sample.bluetooth.peripheral_hr -p siwx917_rb4338a -v --inline-logs -K
+          west twister -v --inline-logs -K \
+            -p siwx917_rb4338a \
+            -s sample.bluetooth.peripheral_hr \
+            -s sample.bluetooth.observer
 
       - name: Build Rail samples
         working-directory: zephyr-silabs