Hybrid mirror: support both flat and $repo/os/$arch layouts

CI: packages go into repo/antergos-pkgs/os/x86_64/ (standard layout),
then flat copies are made at repo/ root for backward compat.
Mirrorlist now has both Server lines, tried in order by pacman.

.github/workflows/build.yml

@@ -38,14 +38,19 @@ jobs:
 
     - name: Set up repo
       run: |
-        mkdir -p repo
-        cp /tmp/pkgout/*.pkg.tar.zst repo/
+        mkdir -p repo/antergos-pkgs/os/x86_64
+        cp /tmp/pkgout/*.pkg.tar.zst repo/antergos-pkgs/os/x86_64/
 
     - name: Generate repo database
       run: |
-        cd repo
+        cd repo/antergos-pkgs/os/x86_64
         repo-add antergos-pkgs.db.tar.gz *.pkg.tar.zst
 
+    - name: Copy to root for flat-URL compat
+      run: |
+        cp repo/antergos-pkgs/os/x86_64/*.pkg.tar.zst repo/
+        cp repo/antergos-pkgs/os/x86_64/antergos-pkgs.db* repo/
+
     - name: Generate index and package list
       run: python generate-index.py