ci: Look for changed packages in the PR branch only

Fixes https://github.com/openwrt/packages/issues/15267.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>
(cherry picked from commit 6b2294ac37)

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

@@ -42,7 +42,7 @@ jobs:
         run: |
           # only detect packages with changes
           PKG_ROOTS=$(find . -name Makefile | grep -v ".*/src/Makefile" | sed -e 's@./\(.*\)/Makefile@\1/@')
-          CHANGES=$(git diff --diff-filter=d --name-only origin/$BRANCH)
+          CHANGES=$(git diff --diff-filter=d --name-only origin/$BRANCH...)
 
           for ROOT in $PKG_ROOTS; do
             for CHANGE in $CHANGES; do