ci: upgrade openwrt/gh-action-sdk workflow to v7

With the change of the Docker container not including pre-built
binaries [1], the previous version of the workflow fails as it still
expects those to be present in the upstream container while the new
versions runs a file called setup.sh if present to configure the
container.

By upgrading openwrt/gh-action-sdk workflow to v7, we make sure that
setup.sh is run and configures the container [2].

[1] 9b55784b18
[2] 0c00b28c11

Signed-off-by: Daniel Nilsson <dannil+github@protonmail.com>

.github/workflows/build.yml

@@ -50,7 +50,7 @@ jobs:
           echo "PACKAGES=$PACKAGES" >> $GITHUB_ENV
 
       - name: Build
-        uses: openwrt/gh-action-sdk@v5
+        uses: openwrt/gh-action-sdk@v7
         env:
           ARCH: ${{ matrix.arch }}-${{ env.BRANCH }}
           FEEDNAME: packages_ci