ci: set up golangci lint in boxo

[galargh]

https://github.com/ipdxco/unified-github-workflows/releases/tag/v1.0.24 added support for golangci-lint to the go-check workflow.

Now, if .golangci.yml is present in a repository, go-check will additionally execute golangci-lint. In PRs, golangci-lint will be check the diff with the target branch (unless that same PR modifies .golangci.yml or .github/workflows/go-check.yml), while on push events, it will check the entire codebase.

In this PR, I added golangci-lint config to this repository. With the added config, go-check will start running the following linters/formatters:

• gci
• gofumpt
• ineffassign
• misspell
• unconvert
• unused
• usestdlibvars

This is a list that was initially proposed in #797

A full list of available linters/formatters can be found at:

• https://golangci-lint.run/usage/linters/
• https://golangci-lint.run/usage/formatters/

I also fixed the issues reported by golangci-lint so that the PR can be cleanly merged.