Add back apt-cacher-ng support for faster builds

- Restore apt-cacher-ng detection and configuration like composefs CI
- Use proxy sources when apt-cacher-ng is available (192.168.1.101:3142)
- Fallback to standard Debian mirrors when apt-cacher-ng is not available
- This should resolve the GPG key verification issues
- Follows the same pattern as the working composefs CI

.forgejo/workflows/ci.yml

@@ -27,10 +27,19 @@ jobs:
         run: |
           echo "Setting up Debian environment for building..."
           
-          # Add Debian repositories
-          echo "Adding Debian unstable repositories..."
+          # Try apt-cacher-ng first, fallback to Debian's automatic mirror selection
+          echo "Checking for apt-cacher-ng availability..."
+
+          # Quick check with timeout to avoid hanging
+          if timeout 10 curl -s --connect-timeout 5 http://192.168.1.101:3142/acng-report.html > /dev/null 2>&1; then
+            echo "✅ apt-cacher-ng is available, configuring proxy sources..."
+            echo "deb http://192.168.1.101:3142/ftp.debian.org/debian unstable main" | sudo tee /etc/apt/sources.list.d/debian-unstable.list
+            echo "deb-src http://192.168.1.101:3142/ftp.us.debian.org/debian unstable main" | sudo tee -a /etc/apt/sources.list.d/debian-unstable.list
+          else
+            echo "⚠️ apt-cacher-ng not available, using standard Debian mirrors..."
             echo "deb http://deb.debian.org/debian unstable main" | sudo tee /etc/apt/sources.list.d/debian-unstable.list
             echo "deb-src http://deb.debian.org/debian unstable main" | sudo tee -a /etc/apt/sources.list.d/debian-unstable.list
+          fi
 
           # Add Debian keyring
           sudo apt update