ci: slowdown due to golangci-lint install

[mmcloughlin]

In order to fix lint issues in #239 it was necessary to switch the golangci-lint install from a release to go install at a specific version. This has the effect of significantly slowing down bootstrap, which in turn slows down the test and lint CI jobs.

Fix this either by switching back to a golangci-lint release when available (must contain the latest staticcheck) or by using Github Actions caching to cache the Go modules and build caches.