CI: multi-arch-test-build: fix error in detecting changed packages

Fix error in detecting changed packages if the packge are present in the
root directory instead of a subdirectory.

This is the case for routing feeds that have packages directly in the
root directory.

This caused a problem in detecting the packages as the sed regex didn't
account for this.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

Author: Ansuel

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

@@ -77,7 +77,7 @@ jobs:
           for ROOT in $PKG_ROOTS; do
             for CHANGE in $CHANGES; do
               if [[ "$CHANGE" == "$ROOT"* ]]; then
-                PACKAGES+=$(echo "$ROOT" | sed -e 's@.*/\(.*\)/@\1 @')
+                PACKAGES+=$(echo "$ROOT" | sed -e 's@\(.*/\)*\(.*\)/@\2 @')
                 break
               fi
             done