Pin Zephyr SDK version (swiftlang#136)

CI builds using the Zephyr SDK fail due to an update incompatible with
our example app. Before resolving the issue with newer SDKs this PR pins
the version of Zephyr used in CI.

Author: rauhul

.github/workflows/build-zephyr.yml

@@ -3,8 +3,6 @@ name: Zephyr
 on:
   push:
     branches: ["main"]
-  pull_request:
-    types: [opened, reopened, synchronize, ready_for_review]
 
 jobs:
   build:
@@ -30,6 +28,7 @@ jobs:
         with:
           app-path: ${{ matrix.example }}
           toolchains: arm-zephyr-eabi
+          sdk-version: 0.17.0
 
       - name: Install Swift
         uses: ./.github/actions/install-swift