ci: run sim and mynewt also on master branch

Sim and Mynewt workflows were only running on pull requests, while they
should also run when new commits are pushed to master.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

.github/workflows/mynewt.yaml

@@ -1,5 +1,9 @@
 # For development, trigger this on any push.
-on: [pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 
 name: Mynewt
 

.github/workflows/sim.yaml

@@ -1,5 +1,9 @@
 # For development, trigger this on any push.
-on: [pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 
 name: Sim