Improve CI script

Upgrade nixbuild-action to put back spaces in the workflow name.
Improve the script for retrieving the result, thanks to @rickynils.

.github/workflows/make-apk.yml

@@ -1,4 +1,4 @@
-name: Build-debug-apk
+name: Build debug apk
 
 on:
   workflow_dispatch:
@@ -14,7 +14,7 @@ jobs:
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: Setup nixbuild.net
-      uses: nixbuild/nixbuild-action@v8
+      uses: nixbuild/nixbuild-action@v9
       with:
         nixbuild_ssh_key: ${{ secrets.nixbuild_ssh_key }}
     - name: Checkout repo
@@ -31,11 +31,15 @@ jobs:
         nix-shell ./shell.nix --run 'make debug.keystore'
     - name: Build
       run: |
-        nix build --eval-store auto --store ssh-ng://eu.nixbuild.net \
-          -f ./shell.nix debug-apk
-        nix build -f ./shell.nix debug-apk
+        store=ssh-ng://eu.nixbuild.net
+        result=$(
+          nix build --json --eval-store auto --store $store \
+            -f ./shell.nix debug-apk \
+          | jq -r '.[0].outputs.out'
+        )
+        nix copy --from $store $result
         mkdir -p _build
-        cp result/*.apk _build
+        cp $result/*.apk _build
     - name: Save debug apk
       uses: actions/upload-artifact@v2
       with: