CI: Disable nightly builds on forks

[mwestphal]

Context:

F3D has an elaborate CI, including a nightly workflow. However, this workflow is caried over to forks, which can be problematic to some. Lets not do that.

Suggested Solution:

• Add a check in nightly workflow that check that the workflow is run on f3d-app/f3d

Skills:

• Github actions

[mwestphal]

@kidharb

[mwestphal]

done.