CI: kernel: Checkout feeds from github

CI: kernel: Checkout feeds from github

Instead of cloning the feeds from the default location at
git.openwrt.org use the github action to clone them directly from
github. We saw some error messages when cloning from git.openwrt.org,
probably related to some rate limiting applied. Cloning from github
within a github action should work more stable.

The "./scripts/feeds update -a" script will use the already checked out
feed repositories and not clone them again from git.openwrt.org, but it
will also not change the branch name.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>