ci: fix nightly image sha
ci: fix nightly image sha

`env.GITHUB_SHA` is not available for action parameters, `github.sha`
should work in this context.

Signed-off-by: Maciej Kwiek <33c1fdf481c8e628d4c6db7ea8dc77f49f2fa5d7@isovalent.com>

tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium