Go: Add '*.exe~' to .gitignore

When you build a Go executable, which does already exists and is in use on Windows (e.g. running) then the output binary name will be extended by a '~' prefix.
https://github.com/golang/go/blob/558eeb2d850d064b0b02b65a7bf3af6c108c248d/src/cmd/go/internal/work/exec.go#L1165-L1170