Enhance Github Action creation to incorporate custom steps

[linusha]

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.