ci: grant contents:read so checkout works on this private fork

.github/workflows/build.yml

@@ -7,7 +7,11 @@ on:
   push:
     branches: [ develop ]
 
-permissions: {}
+permissions:
+  # Private fork: actions/checkout needs contents:read to clone. Upstream is public,
+  # where an empty token can still read the repo, so an empty permissions block works there and
+  # fails here with `fatal: repository not found`.
+  contents: read
 
 # Enrich gradle.properties for CI/CD
 env:
@@ -19,6 +23,9 @@ jobs:
     name: Build APKs
     runs-on: ubuntu-latest
     permissions:
+      # A job-level block replaces the workflow-level one, so contents:read has to be
+      # repeated here or checkout fails on this private fork.
+      contents: read
       # For NejcZdovc/comment-pr
       pull-requests: write
     strategy: