go.mod: update xerrors to a985d3407aa7
xerrors@1f06c39b4373 is can't compile on Go devel.
Because errors.Frame and errors.Formatter were removed Go runtime.
ref:
- https://golang.org/cl/176997
- https://golang.org/cl/177379
$ go get -u -d golang.org/x/xerrors@a985d3407aa7
$ go mod tidy
Change-Id: I55502aabe2f6f384598d006ae5d20748515fa500
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3061
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
authored by