We currently always build kernel packages for the primary channel, which
will usually be systemd-edge. This will cause sideload to fail since it
correctly only looks in the edge repo.

Update envkernel to get the proper channel name and put the kernel
there, so they will always be in the local edge repo and not the
systemd-edge one.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Part-of: !2572