Install gnupg package for Codecov GPG signature verification

- Add gnupg to package installation in Coverage job
- Fixes "gpg: not found" error during Codecov action execution
- Enables proper signature verification for security

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: kkaefer

.github/workflows/build.yml

@@ -92,7 +92,7 @@ jobs:
 
       - name: Install dependencies
         run: |
-          apt-get install -y cmake ninja-build g++ xorg-dev lcov
+          apt-get install -y cmake ninja-build g++ xorg-dev lcov gnupg
           g++ --version
           cmake --version
           ninja --version