Update .golangci.yaml and fixed nil pointer dereference.

[mitchell-as]

DX-1509 Research a linter that prevents us from committing new changes with unhandled nil pointers

The staticcheck linter is already enabled by default, and so is SA5011 (Possible nil pointer dereference), which appears to do what we need:

internal\runbits\runtime\rationalize.go:21:30: SA5011: possible nil pointer dereference (staticcheck)
        isUpdateErr := errs.Matches(*rerr, &ErrUpdate{})

This PR updates the linter with the termtest refactor and disables warnings about constructing structs with unkeyed fields. It also fixes the nil pointer issue found.