cmd/cue: add -C flag, like cmd/go

[myitcv]

Is your feature request related to a problem? Please describe.

See golang/go#50332 for history/context with cmd/go.

Right now, it's not possible to run cmd/cue from one directory, but have it perform commands in the context of another. This is easily solved using cd, or changing a process' working directory... but it's annoying to have to do that.

tar is perhaps one of the most classic examples that uses -C. cmd/go added this flag in https://go-review.googlesource.com/c/go/+/421436.

Describe the solution you'd like

Add a -C flag to cmd/cue, like it was added for cmd/go in https://go-review.googlesource.com/c/go/+/421436.

Describe alternatives you've considered

Per above. Manually having to change directory around the use of cmd/cue.

Additional context

See #1899 for prior discussion.