CI: Pin SDK version to 1.5.1.

This should have been done forever ago!

.github/workflows/cmake.yml

@@ -7,6 +7,7 @@ on:
 env:
   # Customize the CMake build type here (Release, Debug, RelWithDebInfo, etc.)
   BUILD_TYPE: Release
+  PICO_SDK_VERSION: 1.5.1
 
 jobs:
   build:
@@ -43,6 +44,7 @@ jobs:
       uses: actions/checkout@v4
       with:
         repository: raspberrypi/pico-sdk
+        ref: ${{ env.PICO_SDK_VERSION }}
         path: pico-sdk
         submodules: true