Cache golangci-lint cache and Go build cache between runs on CI.

[charleskorn]

What this PR does

This PR introduces caching for golangci-lint's cache and the Go build cache to the CI build.

This improves the performance of each of the affected jobs if a cache hit occurs:

• the build job is around 50% faster
• the linting job is around 60% faster
• the unit test job is around 30% faster
• the integration test jobs vary, but most are ~90s faster

Overall, this improves CI run time by around 4 minutes, which is roughly 20%. For example, compare this run where no cache hits occur with this run where all caches are hit.

This uses GitHub Actions' built-in caching support, and so has the cross-branch caching protections described here.

Which issue(s) this PR fixes or relates to

Relates to #5159

Checklist

• [n/a] Tests updated
• [n/a] Documentation added
• [n/a] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[aknuds1]

Please see comments.

[aknuds1]

Please see comments.

[aknuds1]

Please see comments. I see GitHub still submits double reviews :/

[aknuds1]

Also wondering if we shouldn't be caching $GOMODCACHE.

[aknuds1]

Also wondering if we shouldn't be caching $GOMODCACHE. GitHub double review submit 🤦‍♂️

[aknuds1]

LGTM, although I guess we can drop the debugging steps by now? Is the PR otherwise all done, considering it's still a draft?

[charleskorn]

LGTM, although I guess we can drop the debugging steps by now? Is the PR otherwise all done, considering it's still a draft?

Yep, I need to drop the debugging steps, fix the double-caching from setup-go and add caching for GOMODCACHE to the linting job, as that's the only job that downloads modules.

Once I've made those changes, I'll mark this as ready for review.

[aknuds1]

LGTM!