go-test: remove go vet, tweak golangci-lint, change order

[marians]

In this PR I'm mainly removing the separate execution of go vet, as it's redundant with the checks executed by golangci-lint.

Additional changes:

• Set --max-issues-per-linter to 0 (unlimited) in golangci-lint execution
• Using the environment key to set environment variables in a more idiomatic way
• Run go test before golangci-lint (see below)
• Adding a persistent cache for golangci-lint. We'll have to evaluate in practice whether this really speeds up anything.

---

During testing I noticed that golangci-lint can take a relatively long time to end up finding a compilation problem, like a breaking API change in a major dependency upgrade.

To catch problems like this faster, I moved the go test step to happen before the golangci-lint step. This means that compilation problems are already found in the build that happens on go test, and according to my tests with kubectl-gs, this is faster than the golangci-lint execution.

Checklist

• Make yourself familiar with following readme sections:
  • Coding Standards.
  • Development.
  • Design and Goals.
• ⚠️ Update changelog in CHANGELOG.md.

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