The performance benefit the cache brought was due to a broken sources
CDN mirror handling in OpenWrt.
The cache brings no measurable performance benefit. Disable it to slim
down the pipeline steps.
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.
