Fix the nightly ISO build

The workflow has been running ./build_iso.sh, which has not existed for a long
time — the build moved to bin/omarchy-iso-make. Every scheduled run in the last
100 has failed.

Run the real build driver instead, with NO_BOOT_OFFER=1 so it skips the
interactive "Boot this ISO?" prompt at the end, and collect from ./release/
where the ISO actually lands. Drop the outer archlinux container: omarchy-iso-make
starts its own privileged container, so the job only needs Docker on the runner.
Add a disk cleanup step, since a ~6.5GB ISO plus a ~4.5GB offline mirror does not
fit in a stock runner's free space.

Taken from #67 by @karthikkalidas, which was opened against main. main is 143
commits behind and is no longer the default branch, and scheduled workflows only
run from the default branch, so merging it there would have fixed nothing.

Co-Authored-By: Karthik Kalidas <karthik.kalidas@gmail.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/nightly-build.yml

@@ -2,40 +2,34 @@ name: Nightly ISO Build
 
 on:
   schedule:
-    - cron: '1 0 * * *'  # Daily at 00:01 UTC
-  workflow_dispatch:  # Allow manual triggering
+    - cron: "1 0 * * *" # Daily at 00:01 UTC
+  workflow_dispatch: # Allow manual triggering
 
 jobs:
   build-iso:
     runs-on: ubuntu-latest
-
-    container:
-      image: archlinux/archlinux:latest
-      options: --privileged
-    
     steps:
-    - name: Checkout repository
-      uses: actions/checkout@v4
-      
-    - name: Set up Docker Buildx
-      uses: docker/setup-buildx-action@v3
-      
-    - name: Build ISO in Arch Linux container
-      run: |
-        docker run --rm \
-          --privileged \
-          -v "${{ github.workspace }}:/workspace" \
-          -w /workspace \
-          archlinux:latest \
-          bash -c "
-            pacman -Syu --noconfirm && \
-            chmod +x build_iso.sh && \
-            ./build_iso.sh
-          "
-    
-    - name: Upload ISO artifact
-      uses: actions/upload-artifact@v4
-      with:
-        name: omarchy-iso-${{ github.run_number }}
-        path: out/*.iso
-        retention-days: 30
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Free Disk Space
+        uses: jlumbroso/free-disk-space@v1.3.1
+
+      - name: Build ISO
+        run: NO_BOOT_OFFER=1 ./bin/omarchy-iso-make
+
+      - name: Delete previously uploaded ISO artifact
+        uses: philips-labs/action-delete-artifacts@v1.0.0
+        with:
+          workflow: nightly-build.yml
+        continue-on-error: true
+
+      - name: Upload newly built ISO artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: omarchy-iso-${{ github.run_number }}
+          path: ./release/*.iso
+          retention-days: 1