Linters haven't been run recently

[faddat]

While looking into #1633 I discovered that the linters hadn't recently run, I am guessing because golangci.yml wasn't .golangci.yml.

Before adding PFM or changing things I'd like to get the linters running correctly again, to reduce the likelihood of error.

This will take the form of a series of pull requests, so that we only enable one linter per PR and the resulting work is easier to review.

• enable gci and gofumpt linters #1635
• unnecessary conversion #1637
• lint: unused #1638
• ineffectual assignment #1639