fix: resolve final build failures

debian/rules: Escape $HOME for make (use $$HOME)
  - Make interprets $H as variable, $$ escapes it

build-alpine.sh: Install signing public key
  - Copy .abuild/*.rsa.pub to /etc/apk/keys/
  - Fixes UNTRUSTED signature error on index update

build-arch.sh: Use /home/builduser/repo for all paths
  - PKGDIR=/home/builduser/repo/arch-package
  - WORKSPACE_DIR=/home/builduser/repo
  - Fixes permission denied on act cache path

build-alpine.sh

@@ -122,6 +122,9 @@ if [ "$(id -u)" = "0" ]; then
     # Run abuild as builduser in /home/builduser where APKBUILD exists
     su - builduser -c "cd /home/builduser && abuild checksum && abuild -d -F"
     
+    # Install public key to fix UNTRUSTED signature error
+    cp /home/builduser/.abuild/*.rsa.pub /etc/apk/keys/ 2>/dev/null || true
+    
     # Copy APK from builduser packages to releases
     mkdir -p releases
     cp /home/builduser/packages/x86_64/*.apk releases/ 2>/dev/null || cp /home/builduser/packages/*.apk releases/ 2>/dev/null || find /home/builduser/packages -name "*.apk" -exec cp {} releases/ \; 2>/dev/null || true