Fix K3s golang test Cache Thrashing

[dereknola]

Is your feature request related to a problem? Please describe.

The unit test and integration test workflows utilized the default caching provided by the setup-go action. However, due to limitations around cache access this is a non optimal use of the cache. Every PR is generating their own cache and throwing out the actually useful cache provide by master branch. You can see this in the current cache.

Describe the solution you'd like

What actually needs to happen is:

Master run build-k3s on merge commits, and build a cache of golang depedencies. Only master branch can SAVE a new cache entry

All PRs/non master branches can only RESTORE cache entries for golang. This prevents an overflow of the 10GB cache with entries that can never be used again.

Describe alternatives you've considered

Additional context