Add explicit permissions to workflow files

Fixes code scanning alerts for missing workflow permissions
by setting least-privilege contents: read.

Author: rianbk

.github/workflows/publish.yml

@@ -2,6 +2,9 @@ name: Publish Homey App
 on:
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 jobs:
   publish:
     name: Publish Homey App

.github/workflows/validate.yml

@@ -4,6 +4,9 @@ on:
     branches: [main]
   pull_request:
 
+permissions:
+  contents: read
+
 jobs:
   test:
     name: Validate, Lint & Test