CI: build: fix matching for openwrt release branch for toolchain parsing

CI: build: fix matching for openwrt release branch for toolchain parsing

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: e24a1e6f6d7f ("CI: build: add support for external toolchains from stable branch")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>