CI: fix matching for openwrt release branch for container selection

CI: fix matching for openwrt release branch for container selection

The current match logic doesn't handle test for push events related to
stable release (example openwrt-22.03) but only fork with the related
prefix (example openwrt-22.03-fixup)

Fix wrong matching and while at it also add extra checks to other
matching (check if the branch name actually start with the requested
prefix)

Fixes: abe8a4824210 ("CI: build: add support for per branch tools container")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 65c3d19c4b28ccac0d08d916de0ffa4c0e7b3dc2)