ci: match the repo GitHub runner and action versions

Use ubuntu-26.04, actions/checkout v7 and actions/upload-artifact v7,
and cap the cached Nix store at 6G, matching .github/workflows/ci.yml.

.github/workflows/build-next.yml

@@ -12,15 +12,16 @@ env:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     timeout-minutes: 60
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
       - uses: cachix/install-nix-action@13d8dd58da0234aa297dedd986986ccb8e7f3e24 # v31.11.1
-      - uses: nix-community/cache-nix-action@7df957e333c1e5da7721f60227dbba6d06080569 # v7
+      - uses: nix-community/cache-nix-action@7df957e333c1e5da7721f60227dbba6d06080569 # v7.0.2
         with:
           primary-key: nix-v2-${{ hashFiles('flake.lock', '**/*.nix') }}
           restore-prefixes-first-match: nix-v2-
+          gc-max-store-size-linux: 6G
       - name: Build next builds
         run: nix develop --command gradle :app:assembleDebug :app:assembleNext --no-daemon
       - name: Collect next builds
@@ -28,7 +29,7 @@ jobs:
           cp app/build/outputs/apk/debug/app-debug.apk subtracks-next-debug.apk
           cp app/build/outputs/apk/next/app-next.apk subtracks-next.apk
       - name: Upload next builds
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: subtracks-next
           path: |