CI: replace unsupported upload/download-artifact@v4 with shared host dir

Forgejo does not support @actions/artifact v2 (GHES limitation).
Self-hosted jobs share /tmp/ci-artifacts between jobs.

.forgejo/workflows/ci.yml

@@ -18,15 +18,14 @@ jobs:
       - name: Copy firmware to Android assets
         run: bash tools/copy_firmware.sh
 
-      - name: Upload firmware artifact
-        uses: actions/upload-artifact@v4
-        with:
-          name: firmware
-          path: |
-            .pio/build/m5stack-atoms3r/firmware.bin
-            .pio/build/m5stack-atoms3r/bootloader.bin
-            .pio/build/m5stack-atoms3r/partitions.bin
-            android/app/src/main/assets/firmware/boot_app0.bin
+      - name: Stage firmware for android job
+        run: |
+          rm -rf /tmp/ci-artifacts/firmware
+          mkdir -p /tmp/ci-artifacts/firmware
+          cp .pio/build/m5stack-atoms3r/firmware.bin /tmp/ci-artifacts/firmware/
+          cp .pio/build/m5stack-atoms3r/bootloader.bin /tmp/ci-artifacts/firmware/
+          cp .pio/build/m5stack-atoms3r/partitions.bin /tmp/ci-artifacts/firmware/
+          cp android/app/src/main/assets/firmware/boot_app0.bin /tmp/ci-artifacts/firmware/
 
   android:
     needs: firmware
@@ -34,10 +33,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Download firmware
-        uses: actions/download-artifact@v4
-        with:
-          name: firmware
+      - name: Restore firmware from firmware job
+        run: |
+          cp /tmp/ci-artifacts/firmware/boot_app0.bin android/app/src/main/assets/firmware/
+          cp /tmp/ci-artifacts/firmware/firmware.bin android/app/src/main/assets/firmware/
+          cp /tmp/ci-artifacts/firmware/bootloader.bin android/app/src/main/assets/firmware/
+          cp /tmp/ci-artifacts/firmware/partitions.bin android/app/src/main/assets/firmware/
 
       - name: Build debug APK
         working-directory: android
@@ -45,8 +46,7 @@ jobs:
         env:
           ANDROID_HOME: ${{ vars.ANDROID_HOME || '/opt/android-sdk' }}
 
-      - name: Upload debug APK
-        uses: actions/upload-artifact@v4
-        with:
-          name: app-debug
-          path: android/app/build/outputs/apk/debug/app-debug.apk
+      - name: Stage APK
+        run: |
+          mkdir -p /tmp/ci-artifacts
+          cp android/app/build/outputs/apk/debug/app-debug.apk /tmp/ci-artifacts/