This adds the ability to cache OpenWrt dependencies on a per-target
base. Artifacts over 10MB are excluded, as GitHub imposes a limit of 5G
of available space per repository cache. This affects mostly
linux(-firmware) and gcc / gdb.

The goal is to reduce the total amount of requests necessary to fetch
dependencies.

- Rely on shebang instead of setting shell in workflow
- Run whole install-dependencies.sh in sudo
- Use /bin/sh instead of bash
- set -e

The new step is added with `if: ${{ !cancelled() }}`, so the logs are
stored even when the build fails.

This enables build-testing only on master as well as next and release
branches.