ci: mynewt: Disable run on PR and add daily cron

Only build on push and main branch daily (until mynewt integration
is fixed).

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>

Author: sjanc

.github/workflows/mynewt.yaml

@@ -3,7 +3,8 @@ on:
   push:
     branches:
       - main
-  pull_request:
+  schedule:
+    - cron: 42 3 * * *
 
 name: Mynewt