feat(build): Allow specifying snippets for a build.

* Allow using snippets https://docs.zephyrproject.org/latest/build/snippets/using.html
  for user builds in a `snippets` array properly.

.github/workflows/build-user-config.yml

@@ -65,6 +65,7 @@ jobs:
           board: ${{ matrix.board }}
           shield: ${{ matrix.shield }}
           artifact_name: ${{ matrix.artifact-name }}
+          snippet: ${{ matrix.snippet }}
         run: |
           if [ -e zephyr/module.yml ]; then
             export zmk_load_arg=" -DZMK_EXTRA_MODULES='${GITHUB_WORKSPACE}'"
@@ -75,7 +76,12 @@ jobs:
             echo "base_dir=${GITHUB_WORKSPACE}" >> $GITHUB_ENV
           fi
 
+          if [ -n "${snippet}" ]; then
+            extra_west_args="-S \"${snippet}\""
+          fi
+
           echo "zephyr_version=${ZEPHYR_VERSION}" >> $GITHUB_ENV
+          echo "extra_west_args=${extra_west_args}" >> $GITHUB_ENV
           echo "extra_cmake_args=${shield:+-DSHIELD=\"$shield\"}${zmk_load_arg}" >> $GITHUB_ENV
           echo "display_name=${shield:+$shield - }${board}" >> $GITHUB_ENV
           echo "artifact_name=${artifact_name:-${shield:+$shield-}${board}-zmk}" >> $GITHUB_ENV
@@ -120,7 +126,7 @@ jobs:
       - name: West Build (${{ env.display_name }})
         working-directory: ${{ env.base_dir }}
         shell: sh -x {0}
-        run: west build -s zmk/app -d "${{ env.build_dir }}" -b "${{ matrix.board }}" -- -DZMK_CONFIG=${{ env.base_dir }}/${{ inputs.config_path }} ${{ env.extra_cmake_args }} ${{ matrix.cmake-args }}
+        run: west build -s zmk/app -d "${{ env.build_dir }}" -b "${{ matrix.board }}" ${{ env.extra_west_args }} -- -DZMK_CONFIG=${{ env.base_dir }}/${{ inputs.config_path }} ${{ env.extra_cmake_args }} ${{ matrix.cmake-args }}
 
       - name: ${{ env.display_name }} Kconfig file
         run: |