cmd/go: prevent go work use panic when given a file

cmd/go: prevent go work use panic when given a file

The current implementation fails to identify that an argument to go work
use is a file when expecting a directory, and panics when attempting to
access it as a directory. This change checks arguments are directories
and generates an error otherwise.

Fixes #51749

Change-Id: If8f69d233409e93fcf391a8774bace74c031c986
Reviewed-on: https://go-review.googlesource.com/c/go/+/393615
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>