cmd/go: restore default vet analyzers for targets in GOROOT

This fixes a regression introduced in CL 209498,
found while investigating #32471.

Also fix $WORK replacement in cmd/go/internal/work.(*Builder).Showcmd
when b.WorkDir includes a backslash and appears in a quoted string.
That fix is needed in order to write a precise test that passes under Windows,
since Windows directories nearly always include backslashes.

Updates #35837

Change-Id: I5fddc5435d5d283a3e598989209d873b59b0a39c
Reviewed-on: https://go-review.googlesource.com/c/go/+/210937


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>