Go 1.16 CI is failing

[jdolitsky]

Even in PRs not modifying Go source, fails with:

Error: ../../../../pkg/mod/honnef.co/go/tools@v0.3.1/go/ir/builder.go:36:2: //go:build comment without // +build comment
Error: make: *** [Makefile:111: .install.lint] Error 1
Error: Process completed with exit code 2.

[sudo-bmitch]

Looks like go get github.com/golangci/golangci-lint/cmd/golangci-lint@latest is no longer working with Go 1.16, and I suspect they won't fix that now that 1.18 is out. So our choice is to either cut support for 1.16, or pin to an older version of the linter. I opted for the latter.