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>

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

@@ -68,7 +68,7 @@ jobs:
           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