feat: offer the other card in offline launchers

Without a network, muOS and ArkOS launchers ask to copy the BIOS files from the card the system does not read (muOS bind.sh, dArkOS roms2), then install with --from and the pack. CI checks that shellcheck is present, and the test fails under CI without it.

.github/workflows/validate.yml

@@ -227,6 +227,11 @@ jobs:
       - name: Hold install.py to Python 3.8
         run: vermin --no-tips -t=3.8- --violations install.py
 
+      # The handheld launchers' test runs shellcheck and fails under CI
+      # without it. The Ubuntu image ships it: this names it if one stops.
+      - name: Check that shellcheck is installed
+        run: shellcheck --version
+
       # The site is built as deploy-site.yml builds it, from the lockfile.
       - uses: actions/setup-node@949feb2413d6458794dcd2491c4babbbce0c15c1  # v7.1.0
         with: