CI: build: add support for external toolchains from stable branch

CI: build: add support for external toolchains from stable branch

Add support to use external toolchains from stable branch if we are
testing commit targeting stable openwrt branch in kernel and packages
workflow.

With pr the target branch is parsed and the right toolchain is used.

To use the stable toolchain for local testing the branch needs to have
the prefix openwrt-[0-9][0-9].[0-9][0-9]- (example openwrt-21.02-fixup)

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit e24a1e6f6d7f08fb766eb11b8008f8fc5b72d072)