ci: disable nightly on forks (#275)

diff --git a/.github/workflows/nightly.yml b/.github/workflows/nightly.yml
index 7ce8571..a358c78 100644
--- a/.github/workflows/nightly.yml
+++ b/.github/workflows/nightly.yml
@@ -35,6 +35,8 @@
 
 jobs:
   build:
+    if: github.repository == 'apache/cordova-coho'
+
     runs-on: ubuntu-latest
 
     env: