add preem bincache

.forgejo/workflows/build.yaml

@@ -15,7 +15,14 @@ jobs:
         # keep-sorted end
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v4
-      - run: nix build --no-link .#nixosConfigurations.${{ matrix.host }}.config.system.build.toplevel
+      - run: nix build .#nixosConfigurations.${{ matrix.host }}.config.system.build.toplevel
+      - run: |
+          nix shell --inputs-from . nixpkgs#attic-client -c sh -c '
+            attic-client login preem https://preem-bincache.trollhive.monster "$PREEM_PUSH_TOKEN"
+            attic-client push preem:grid ./result
+          '
+        env:
+          PREEM_PUSH_TOKEN: ${{ secrets.PREEM_PUSH_TOKEN }}
 
   # nightly-flake-update.yaml opens PRs from flake-update-* branches; if
   # the build fails on one of those, close it rather than leaving a