ci: fold the nightly artifact into the CI job

Drop the separate build-nightly workflow: on a push to main the CI job
uploads the debug and nightly APKs as the subtracks-nightly artifact, so
the nightly variant is built once instead of twice (and there is no
second Nix/cache setup).

.github/workflows/build-nightly.yml

@@ -1,34 +0,0 @@
-name: Build Nightly
-
-on:
-  push:
-    branches: [main]
-
-permissions:
-  contents: read
-
-jobs:
-  build:
-    runs-on: ubuntu-26.04
-    timeout-minutes: 60
-    permissions:
-      contents: read
-    steps:
-      - uses: actions/checkout@v7
-        with:
-          persist-credentials: false
-      - uses: ./.github/actions/build
-        with:
-          gradle-args: ":app:assembleDebug :app:assembleNightly --no-daemon"
-      - name: Collect
-        run: |
-          cp app/build/outputs/apk/debug/app-debug.apk subtracks-nightly-debug.apk
-          cp app/build/outputs/apk/nightly/app-nightly.apk subtracks-nightly.apk
-      - name: Checksums
-        run: sha256sum subtracks-nightly.apk subtracks-nightly-debug.apk
-      - uses: actions/upload-artifact@v7
-        with:
-          name: subtracks-nightly
-          path: |
-            subtracks-nightly.apk
-            subtracks-nightly-debug.apk

.github/workflows/ci.yml

@@ -23,7 +23,22 @@ jobs:
           persist-credentials: false
       - uses: ./.github/actions/build
         with:
-          gradle-args: ":app:ktlintCheck :app:lintDebug :app:testDebugUnitTest :app:assembleNightly --no-configuration-cache --console=plain --no-daemon"
+          gradle-args: ":app:ktlintCheck :app:lintDebug :app:testDebugUnitTest :app:assembleDebug :app:assembleNightly --no-configuration-cache --console=plain --no-daemon"
+      - name: Collect nightly builds
+        if: github.ref == 'refs/heads/main'
+        run: |
+          cp app/build/outputs/apk/debug/app-debug.apk subtracks-nightly-debug.apk
+          cp app/build/outputs/apk/nightly/app-nightly.apk subtracks-nightly.apk
+      - name: Checksums
+        if: github.ref == 'refs/heads/main'
+        run: sha256sum subtracks-nightly.apk subtracks-nightly-debug.apk
+      - uses: actions/upload-artifact@v7
+        if: github.ref == 'refs/heads/main'
+        with:
+          name: subtracks-nightly
+          path: |
+            subtracks-nightly.apk
+            subtracks-nightly-debug.apk
       - uses: actions/cache@v6
         with:
           path: .integration/music