Enhance Github Action creation to incorporate custom steps

Currently, we replace the entirety of the actions. This leads to the need to create custom actions, which however need to be updated by hand, which is especially cumbersome once one has a custom build action and needs to update the lively commit hash by hand.
A solution could be to markup custom sections in the actions with comments, which we could parse and carry over to a new version. Alternatively, we could just replace the commit hashes, however, this has the obvious downside of not carrying over improvements such as those introduced in #1651. I still need to think about this.