Update CI artifact paths for new self-hosted runner

The build-prod and build-dev workflows wrote their ISO/image artifacts to paths tied to the old 'build-server' runner:

- build-prod.yml: /home/runner/radiantwave
- build-dev.yml: /mnt/actions_build_storage/outputs

Both now stage, copy, and clean up artifacts under the new runner's home directory instead:

- build-prod.yml: /home/user/radiantwave
- build-dev.yml: /home/usr/radiantwave

No other behavior changes; the output files are still scp'd to the fileserver and the flake.lock commit step is unchanged.

.gitea/workflows/build-dev.yml

@@ -49,7 +49,7 @@ jobs:
           nix flake update radiantwave-dev
 
       - name: Prepare output directory
-        run: mkdir -p /mnt/actions_build_storage/outputs
+        run: mkdir -p /home/usr/radiantwave
 
       - name: Build dev ISO
         run: |
@@ -61,16 +61,16 @@ jobs:
         run: |
           ISO_DATE=$(date +%Y-%m-%d_%H-%M-%S)
           FILENAME="radiantwave-kiosk-iso-dev-${ISO_DATE}.iso"
-          cp -L result/iso/*.iso "/mnt/actions_build_storage/outputs/${FILENAME}"
+          cp -L result/iso/*.iso "/home/usr/radiantwave/${FILENAME}"
           echo "DEV_ISO_FILENAME=${FILENAME}" >> "$GITHUB_ENV"
 
       - name: Copy dev ISO to fileserver
         run: |
-          scp "/mnt/actions_build_storage/outputs/${DEV_ISO_FILENAME}" \
+          scp "/home/usr/radiantwave/${DEV_ISO_FILENAME}" \
             radiantwave-infra:/mnt/infrastructure_data/files/images/
 
       - name: Cleanup dev ISO
-        run: rm -f "/mnt/actions_build_storage/outputs/${DEV_ISO_FILENAME}"
+        run: rm -f "/home/usr/radiantwave/${DEV_ISO_FILENAME}"
 
       - name: Commit updated flake.lock
         run: |