ci: use openwrt/gh-action-sdk@v4

In order to use feeds from GH mirror for GH actions, thus saving a lot
of resources being wasted. While at it fix whitespace issue.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
[applied to routing feed]
[whitespace issue not existing in routing feed]
Signed-off-by: Nick Hainke <vincent@systemli.org>
(cherry picked from commit 5702d2e40b)

.github/workflows/multi-arch-test-build.yml

@@ -79,7 +79,7 @@ jobs:
           echo "PACKAGES=$PACKAGES" >> $GITHUB_ENV
 
       - name: Build
-        uses: openwrt/gh-action-sdk@v3
+        uses: openwrt/gh-action-sdk@v4
         env:
           ARCH: ${{ matrix.arch }}-${{ env.BRANCH }}
           FEEDNAME: packages_ci