chore(build): disable nightly workflow in forks

[hydrosquall]

Motivation

Noticed after forking dendron that I was getting a github actions error message once per evening about a failing publish job.

Commit

• make sure your branch names adhere to our branch style
• make sure the commit message follows Dendron's commit style
• if this pull request is addressing an existing issue, make sure to link this PR to the issue that it is resolving.
  • Fixes build: github actions shouldn't run in forks of the dendron monorepo #3674

Code

• code should follow Code Conventions
• sticking to existing conventions instead of creating new ones

Tests

N/A, doesn't modify application code .

Docs

• if your change reflects documentation changes, also submit a PR to dendron-site and mention the doc PR link in your current PR (NOTE: submit the PR against the dev branch of the dendron-site repo) (no)

• does this change introduce a new or better way of doing things that others need to be aware of? if so, an async should be created and a process added in Development or Packages (no)

Notes

• Credit for the fix comes from Do not run GitHub Action Workflows on fork of example repository prisma/prisma#3539 (comment)

[kevinslin]

nice catch. thanks for the fix!