* Expire cache periodically to avoid unbounded size
The cache key includes a sequence number that rotates every 7 days
but because we are also using the base `golangci-lint.cache` as a
restore key, the new cache will always be seeded with the full
contents of the old cache.
In particular for the go module cache, this leads to an ever
increasing number of cached packages that never get pruned.
This commit updates it so we stop using `golangci-lint.cache` as
a restore key, which will force a build from an empty cache once
every 7 days.
* Rebuild files in dist/
Add option to skip caching the Go package directory (~/go/pkg).
Add option to skip caching the Go build directory (~/.cache/go-build).
Update README to mention new options.
