ci: Only do cron job on main repository

I noticed my doanac/meta-qcom-hwe fork was trying to run this cron job
every night but fails because there is no runner for it.

Signed-off-by: Andy Doan <andy@foundries.io>

.github/workflows/nightly-build.yml

@@ -8,6 +8,7 @@ on:
 
 jobs:
   compile:
+    if: github.repository == 'quic-yocto/meta-qcom-hwe'
     strategy:
       fail-fast: true
       matrix: