ci: restrict default GITHUB_TOKEN permissions to contents: read

None of these workflows write through GITHUB_TOKEN: artifact upload,
Firebase, S3, Play Store, gh-pages and the Appium dispatch all use their
own credentials. Reusable workflows they call inherit the same cap.

.github/workflows/pull_request.yml

@@ -11,6 +11,9 @@ on:
         type: string
 
 
+permissions:
+  contents: read
+
 jobs:
   test:
     uses: pezkuwichain/pezkuwi-wallet-android/.github/workflows/android_build.yml@main